text: Title: tox 3.27.1 to 4.0.9, environment var substitution failing
  Body: ## Issue
  
  All of our build pipelines are failing due to what appears to be a bug with environment variable substitution in our tox.ini.  The variable substitution works as expected with tox==3.27.1, but we start having issues with tox==4.0.9.
  
  ## Environment
  
  Provide at least:
  
  - OS: Windows
  - `pip list` of the host Python where `tox` is installed:
  
  ```console
  (.venv) C:\tmp\tox_4.0.9_bug>pip list
  Package       Version
  ------------- -------
  cachetools    5.2.0
  chardet       5.1.0
  colorama      0.4.6
  distlib       0.3.6
  filelock      3.8.2
  packaging     22.0
  pip           22.3.1
  platformdirs  2.6.0
  pluggy        1.0.0
  pyproject_api 1.2.1
  setuptools    65.6.3
  tomli         2.0.1
  tox           4.0.9
  virtualenv    20.17.1
  wheel         0.38.4
  ```
  
  ## Output of running tox
  
  Provide the output of `tox -rvv`:
  
  ```console
  (.venv) C:\tmp\tox_4.0.9_bug>tox -rvv
  default: 2108 I find interpreter for spec PythonSpec(path=C:\tmp\tox_4.0.9_bug\.venv\Scripts\python.exe) [virtualenv\discovery\builtin.py:56]
  default: 2134 D got python info of C:\Users\nws2293\AppData\Local\Programs\Python\Python39\python.exe from C:\Users\nws2293\AppData\Local\pypa\virtualenv\py_info\1\72aa99cb99a91779d7fa96aaf09f6d3b1207abe0138f99ab0af57c8a95fa9102.json [virtualenv\app_data\via_disk_folder.py:129]
  default: 2173 I proposed PythonInfo(spec=CPython3.9.10.final.0-64, system=C:\Users\nws2293\AppData\Local\Programs\Python\Python39\python.exe, exe=C:\tmp\tox_4.0.9_bug\.venv\Scripts\python.exe, platform=win32, version='3.9.10 (tags/v3.9.10:f2f3f53, Jan 17 2022, 15:14:21) [MSC v.1929 64 bit (AMD64)]', encoding_fs_io=utf-8-cp1252) [virtualenv\discovery\builtin.py:63]
  default: 2174 D accepted PythonInfo(spec=CPython3.9.10.final.0-64, system=C:\Users\nws2293\AppData\Local\Programs\Python\Python39\python.exe, exe=C:\tmp\tox_4.0.9_bug\.venv\Scripts\python.exe, platform=win32, version='3.9.10 (tags/v3.9.10:f2f3f53, Jan 17 2022, 15:14:21) [MSC v.1929 64 bit (AMD64)]', encoding_fs_io=utf-8-cp1252) [virtualenv\discovery\builtin.py:65]
  default: 2203 D symlink on filesystem does not work [virtualenv\info.py:43]
  default: 2207 D filesystem is not case-sensitive [virtualenv\info.py:24]
  default: 2420 I create virtual environment via CPython3Windows(dest=C:\tmp\tox_4.0.9_bug\.tox\default, clear=False, no_vcs_ignore=False, global=False) [virtualenv\run\session.py:48]
  default: 2422 D create folder C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages [virtualenv\util\path\_sync.py:9]
  default: 2427 D create folder C:\tmp\tox_4.0.9_bug\.tox\default\Scripts [virtualenv\util\path\_sync.py:9]
  default: 2429 D write C:\tmp\tox_4.0.9_bug\.tox\default\pyvenv.cfg [virtualenv\create\pyenv_cfg.py:30]
  default: 2430 D         home = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 2431 D         implementation = CPython [virtualenv\create\pyenv_cfg.py:34]
  default: 2431 D         version_info = 3.9.10.final.0 [virtualenv\create\pyenv_cfg.py:34]
  default: 2432 D         virtualenv = 20.17.1 [virtualenv\create\pyenv_cfg.py:34]
  default: 2433 D         include-system-site-packages = false [virtualenv\create\pyenv_cfg.py:34]
  default: 2434 D         base-prefix = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 2434 D         base-exec-prefix = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 2434 D         base-executable = C:\Users\nws2293\AppData\Local\Programs\Python\Python39\python.exe [virtualenv\create\pyenv_cfg.py:34]
  default: 2439 D copy C:\Users\nws2293\AppData\Local\Programs\Python\Python39\Lib\venv\scripts\nt\python.exe to C:\tmp\tox_4.0.9_bug\.tox\default\Scripts\python.exe [virtualenv\util\path\_sync.py:36]        
  default: 2448 D copy C:\Users\nws2293\AppData\Local\Programs\Python\Python39\Lib\venv\scripts\nt\pythonw.exe to C:\tmp\tox_4.0.9_bug\.tox\default\Scripts\pythonw.exe [virtualenv\util\path\_sync.py:36]
  default: 2466 D create virtualenv import hook file C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\_virtualenv.pth [virtualenv\create\via_global_ref\api.py:89]
  default: 2473 D create C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\_virtualenv.py [virtualenv\create\via_global_ref\api.py:92]
  default: 2480 D ============================== target debug ============================== [virtualenv\run\session.py:50]
  default: 2481 D debug via 'C:\tmp\tox_4.0.9_bug\.tox\default\Scripts\python.exe' 'C:\tmp\tox_4.0.9_bug\.venv\lib\site-packages\virtualenv\create\debug.py' [virtualenv\create\creator.py:197]
  default: 2480 D {
    "sys": {
      "executable": "C:\\tmp\\tox_4.0.9_bug\\.tox\\default\\Scripts\\python.exe",
      "_base_executable": "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39\\python.exe",
      "prefix": "C:\\tmp\\tox_4.0.9_bug\\.tox\\default",
      "base_prefix": "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39",
      "real_prefix": null,
      "exec_prefix": "C:\\tmp\\tox_4.0.9_bug\\.tox\\default",
      "base_exec_prefix": "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39",
      "path": [
        "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39\\python39.zip",
        "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39\\DLLs",
        "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39\\lib",
        "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39",
        "C:\\tmp\\tox_4.0.9_bug\\.tox\\default",
        "C:\\tmp\\tox_4.0.9_bug\\.tox\\default\\lib\\site-packages"
      ],
      "meta_path": [
        "<class '_virtualenv._Finder'>",
        "<class '_frozen_importlib.BuiltinImporter'>",
        "<class '_frozen_importlib.FrozenImporter'>",
        "<class '_frozen_importlib_external.PathFinder'>"
      ],
      "fs_encoding": "utf-8",
      "io_encoding": "cp1252"
    },
    "version": "3.9.10 (tags/v3.9.10:f2f3f53, Jan 17 2022, 15:14:21) [MSC v.1929 64 bit (AMD64)]",
    "makefile_filename": "C:\\Users\\nws2293\\AppData\\Local\\Programs\\Python\\Python39\\Lib\\config\\Makefile",
    "os": "<module 'os' from 'C:\\\\Users\\\\nws2293\\\\AppData\\\\Local\\\\Programs\\\\Python\\\\Python39\\\\lib\\\\os.py'>",
    "site": "<module 'site' from 'C:\\\\Users\\\\nws2293\\\\AppData\\\\Local\\\\Programs\\\\Python\\\\Python39\\\\lib\\\\site.py'>",
    "datetime": "<module 'datetime' from 'C:\\\\Users\\\\nws2293\\\\AppData\\\\Local\\\\Programs\\\\Python\\\\Python39\\\\lib\\\\datetime.py'>",
    "math": "<module 'math' (built-in)>",
    "json": "<module 'json' from 'C:\\\\Users\\\\nws2293\\\\AppData\\\\Local\\\\Programs\\\\Python\\\\Python39\\\\lib\\\\json\\\\__init__.py'>"
  } [virtualenv\run\session.py:51]
  default: 3034 I add seed packages via FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=C:\Users\nws2293\AppData\Local\pypa\virtualenv) [virtualenv\run\session.py:55]
  default: 3051 D got embed update of distribution setuptools from C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\embed\3\setuptools.json [virtualenv\app_data\via_disk_folder.py:129]
  default: 3052 D got embed update of distribution pip from C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\embed\3\pip.json [virtualenv\app_data\via_disk_folder.py:129]
  default: 3053 D got embed update of distribution wheel from C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\embed\3\wheel.json [virtualenv\app_data\via_disk_folder.py:129]
  default: 3060 D install setuptools from wheel C:\tmp\tox_4.0.9_bug\.venv\lib\site-packages\virtualenv\seed\wheels\embed\setuptools-65.6.3-py3-none-any.whl via CopyPipInstall [virtualenv\seed\embed\via_app_data\via_app_data.py:47]
  default: 3062 D install pip from wheel C:\tmp\tox_4.0.9_bug\.venv\lib\site-packages\virtualenv\seed\wheels\embed\pip-22.3.1-py3-none-any.whl via CopyPipInstall [virtualenv\seed\embed\via_app_data\via_app_data.py:47]
  default: 3063 D install wheel from wheel C:\tmp\tox_4.0.9_bug\.venv\lib\site-packages\virtualenv\seed\wheels\embed\wheel-0.38.4-py3-none-any.whl via CopyPipInstall [virtualenv\seed\embed\via_app_data\via_app_data.py:47]
  default: 3077 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\pip-22.3.1-py3-none-any\pip to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\pip [virtualenv\util\path\_sync.py:36]
  default: 3078 D copy C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\distutils-precedence.pth to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\distutils-precedence.pth [virtualenv\util\path\_sync.py:36]
  default: 3080 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\wheel-0.38.4-py3-none-any\wheel to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\wheel [virtualenv\util\path\_sync.py:36]
  default: 3094 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\pkg_resources to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\pkg_resources [virtualenv\util\path\_sync.py:36]
  default: 3399 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\wheel-0.38.4-py3-none-any\wheel-0.38.4.dist-info to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\wheel-0.38.4.dist-info [virtualenv\util\path\_sync.py:36]
  default: 3515 D copy C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\wheel-0.38.4-py3-none-any\wheel-0.38.4.virtualenv to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\wheel-0.38.4.virtualenv [virtualenv\util\path\_sync.py:36]
  default: 3583 D generated console scripts wheel3.9.exe wheel-3.9.exe wheel3.exe wheel.exe [virtualenv\seed\embed\via_app_data\pip_install\base.py:41]
  default: 3911 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\setuptools to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\setuptools [virtualenv\util\path\_sync.py:36]
  default: 6578 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\setuptools-65.6.3.dist-info to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\setuptools-65.6.3.dist-info [virtualenv\util\path\_sync.py:36]
  default: 6686 D copy C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\setuptools-65.6.3.virtualenv to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\setuptools-65.6.3.virtualenv [virtualenv\util\path\_sync.py:36]
  default: 6693 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\setuptools-65.6.3-py3-none-any\_distutils_hack to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\_distutils_hack [virtualenv\util\path\_sync.py:36]
  default: 6737 D generated console scripts  [virtualenv\seed\embed\via_app_data\pip_install\base.py:41]
  default: 10295 D copy directory C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\pip-22.3.1-py3-none-any\pip-22.3.1.dist-info to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\pip-22.3.1.dist-info [virtualenv\util\path\_sync.py:36]
  default: 10382 D copy C:\Users\nws2293\AppData\Local\pypa\virtualenv\wheel\3.9\image\1\CopyPipInstall\pip-22.3.1-py3-none-any\pip-22.3.1.virtualenv to C:\tmp\tox_4.0.9_bug\.tox\default\Lib\site-packages\pip-22.3.1.virtualenv [virtualenv\util\path\_sync.py:36]
  default: 10413 D generated console scripts pip.exe pip3.9.exe pip-3.9.exe pip3.exe [virtualenv\seed\embed\via_app_data\pip_install\base.py:41]
  default: 10413 I add activators for Bash, Batch, Fish, Nushell, PowerShell, Python [virtualenv\run\session.py:61]
  default: 10513 D write C:\tmp\tox_4.0.9_bug\.tox\default\pyvenv.cfg [virtualenv\create\pyenv_cfg.py:30]
  default: 10513 D        home = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 10514 D        implementation = CPython [virtualenv\create\pyenv_cfg.py:34]
  default: 10514 D        version_info = 3.9.10.final.0 [virtualenv\create\pyenv_cfg.py:34]
  default: 10514 D        virtualenv = 20.17.1 [virtualenv\create\pyenv_cfg.py:34]
  default: 10515 D        include-system-site-packages = false [virtualenv\create\pyenv_cfg.py:34]
  default: 10515 D        base-prefix = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 10516 D        base-exec-prefix = C:\Users\nws2293\AppData\Local\Programs\Python\Python39 [virtualenv\create\pyenv_cfg.py:34]
  default: 10516 D        base-executable = C:\Users\nws2293\AppData\Local\Programs\Python\Python39\python.exe [virtualenv\create\pyenv_cfg.py:34]
  default: 10534 W commands[0]> pytest [tox\tox_env\api.py:417]
  default: 10713 E failed with pytest is not allowed, use allowlist_externals to allow it [tox\session\cmd\run\single.py:54]
    default: FAIL code 1 (8.64 seconds)
    evaluation failed :( (9.83 seconds)
  ```
  
  ## Minimal example
  This is a stripped down tox.ini file, "C:\tmp\tox_4.0.9_bug\tox.ini".
  
  ```ini
  [tox]
  minversion = 3.24
  envlist = default
  isolated_build = True
  
  
  [testenv]
  description = Invoke pytest to run automated tests
  passenv =
      HOME
      SETUPTOOLS_*
  setenv =
      TOXINIDIR = {toxinidir}
  
  
  [testenv:{docs}]
  description =
      docs: Pretend to invoke sphinx-build to build the docs
  skip_install = True
  passenv =
      SETUPTOOLS_*
  setenv =
      DOCSDIR = {toxinidir}{/}docs
      BUILDDIR = {toxinidir}{/}docs{/}_build
      docs: BUILD = html
  commands =
      ; sphinx-build --color -b {env:BUILD} -d "{env:BUILDDIR}{/}doctrees" "{env:DOCSDIR}" "{env:BUILDDIR}{/}{env:BUILD}" {posargs}
      python -c 'print("{env:BUILD} -- {env:BUILDDIR}{/}{env:BUILD}")'
  ```
  
  If I run "tox -e docs" with tox 4.0.9, this is the output. Notice that the substitution of `{env:BUILD}` works in the first part of the line but not in the second part following the `{/}`:
  
  ```console
  (.venv) C:\tmp\tox_4.0.9_bug>tox -e docs
  docs: commands[0]> python -c "print(\"html -- C:\tmp\tox_4.0.9_bug\docs\_build{env:BUILD}\")"
  html -- C:      mp      ox_4.0.9_bug\docs\_build{env:BUILD}
    docs: OK (5.73=setup[5.16]+cmd[0.58] seconds)
    congratulations :) (6.73 seconds)
  ```
  
  If I run "tox -e docs" with tox 3.27.1, this is the output:
  
  ```console
  C:\tmp\tox_4.0.9_bug>tox -e docs
  docs run-test-pre: PYTHONHASHSEED='297'
  docs run-test: commands[0] | python -c 'print("html -- C:\tmp\tox_4.0.9_bug\docs\_build\html")'
  html -- C:      mp      ox_4.0.9_bug\docs\_build\html
    docs: commands succeeded
    congratulations :)
  ```
  