text: Title: RuntimeError: config set has been marked final and cannot be extended
  Body: ## Issue
  
  Describe what's the expected behaviour and what you're observing.
  
  Getting a `RuntimeError: config set has been marked final and cannot be extended` at tox startup
  
  ## Environment
  
  Provide at least:
  
  Better example at: https://github.com/tox-dev/tox/issues/3006#issuecomment-1615980590
  
  - OS: A Gitlab runner using an image based of:
  ```
  FROM registry.<local>/docker/library/python:3.11-bullseye
  
  WORKDIR /
  
  # Include external env vars into build context
  ARG PYPI_ADDRESS
  ARG PYPI_USERNAME
  ARG PYPI_PASSWORD
  
  # Python runtime config
  ENV PYTHONIOENCODING utf-8
  ENV PYTHONUNBUFFERED 1
  ENV PYTHONOPTIMIZE 1
  
  ENV PIP_TRUSTED_HOST ${PYPI_ADDRESS}
  ENV PIP_INDEX https://${PYPI_USERNAME}:${PYPI_PASSWORD}@${PYPI_ADDRESS}/runner/org/
  ENV PIP_INDEX_URL https://${PYPI_USERNAME}:${PYPI_PASSWORD}@${PYPI_ADDRESS}/runner/org/+simple/
  ENV PIP_EXTRA_INDEX_URL https://pypi.org/simple
  
  # Virtualenv stuff
  ENV VIRTUAL_ENV=/venv
  
  RUN curl -o virtualenv.pyz https://bootstrap.pypa.io/virtualenv.pyz
  RUN python3.11 /virtualenv.pyz $VIRTUAL_ENV --pip=embed --setuptools=embed --wheel=embed --no-periodic-update
  
  ENV PATH="$VIRTUAL_ENV/bin:$PATH"
  
  COPY requirements.txt .
  RUN pip --version
  RUN python --version
  RUN pip install --upgrade --cache-dir=.pip -r requirements.txt && rm -rf .pip
  ```
  - requirements.txt contains: 
  
  ```console
  setuptools==67.7.2
  wheel==0.40.0
  pytest==7.3.0
  pytest-cov==4.0.0
  pytest-sugar==0.9.6
  coverage==7.2.5
  pip==23.1.2
  tox==4.5.1
  devpi-client==6.0.4
  ```
  
  (We have seen this in older versions of Tox and Python)
  
  
  ## Output of running tox
  
  Provide the output of `tox -rvv`:
  
  ```console
  tox -vvvvv
  ROOT: 170 D setup logging to NOTSET on pid 30 [tox/report.py:221]
  py311: 255 I find interpreter for spec PythonSpec(major=3, minor=11) [virtualenv/discovery/builtin.py:56]
  py311: 255 I proposed PythonInfo(spec=CPython3.11.2.final.0-64, exe=/usr/local/bin/python, platform=linux, version='3.11.2 (main, Mar 23 2023, 17:12:29) [GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:63]
  py311: 255 D accepted PythonInfo(spec=CPython3.11.2.final.0-64, exe=/usr/local/bin/python, platform=linux, version='3.11.2 (main, Mar 23 2023, 17:12:29) [GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:65]
  py311: 257 D filesystem is case-sensitive [virtualenv/info.py:24]
  py311: 291 I create virtual environment via CPython3Posix(dest=/builds/<org>/<project>/.tox/py311, clear=False, no_vcs_ignore=False, global=False) [virtualenv/run/session.py:48]
  py311: 291 D create folder /builds/<org>/<project>/.tox/py311/bin [virtualenv/util/path/_sync.py:9]
  py311: 291 D create folder /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages [virtualenv/util/path/_sync.py:9]
  py311: 292 D write /builds/<org>/<project>/.tox/py311/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
  py311: 292 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	version_info = 3.11.2.final.0 [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	virtualenv = 20.21.0 [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D 	base-executable = /usr/local/bin/python [virtualenv/create/pyenv_cfg.py:34]
  py311: 292 D symlink /usr/local/bin/python to /builds/<org>/<project>/.tox/py311/bin/python [virtualenv/util/path/_sync.py:28]
  py311: 293 D create virtualenv import hook file /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/_virtualenv.pth [virtualenv/create/via_global_ref/api.py:89]
  py311: 293 D create /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/_virtualenv.py [virtualenv/create/via_global_ref/api.py:92]
  py311: 293 D ============================== target debug ============================== [virtualenv/run/session.py:50]
  py311: 293 D debug via /builds/<org>/<project>/.tox/py311/bin/python /usr/local/lib/python3.11/site-packages/virtualenv/create/debug.py [virtualenv/create/creator.py:193]
  py311: 293 D {
    "sys": {
      "executable": "/builds/<org>/<project>/.tox/py311/bin/python",
      "_base_executable": "/usr/local/bin/python3.11",
      "prefix": "/builds/<org>/<project>/.tox/py311",
      "base_prefix": "/usr/local",
      "real_prefix": null,
      "exec_prefix": "/builds/<org>/<project>/.tox/py311",
      "base_exec_prefix": "/usr/local",
      "path": [
        "/usr/local/lib/python311.zip",
        "/usr/local/lib/python3.11",
        "/usr/local/lib/python3.11/lib-dynload",
        "/builds/<org>/<project>/.tox/py311/lib/python3.11/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": "utf-8"
    },
    "version": "3.11.2 (main, Mar 23 2023, 17:12:29) [GCC 10.2.1 20210110]",
    "makefile_filename": "/usr/local/lib/python3.11/config-3.11-x86_64-linux-gnu/Makefile",
    "os": "<module 'os' (frozen)>",
    "site": "<module 'site' (frozen)>",
    "datetime": "<module 'datetime' from '/usr/local/lib/python3.11/datetime.py'>",
    "math": "<module 'math' from '/usr/local/lib/python3.11/lib-dynload/math.cpython-311-x86_64-linux-gnu.so'>",
    "json": "<module 'json' from '/usr/local/lib/python3.11/json/__init__.py'>"
  } [virtualenv/run/session.py:51]
  py311: 334 I add seed packages via FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/root/.local/share/virtualenv) [virtualenv/run/session.py:55]
  py311: 338 D got embed update of distribution setuptools from /root/.local/share/virtualenv/wheel/3.11/embed/3/setuptools.json [virtualenv/app_data/via_disk_folder.py:129]
  py311: 342 D got embed update of distribution wheel from /root/.local/share/virtualenv/wheel/3.11/embed/3/wheel.json [virtualenv/app_data/via_disk_folder.py:129]
  py311: 342 D got embed update of distribution pip from /root/.local/share/virtualenv/wheel/3.11/embed/3/pip.json [virtualenv/app_data/via_disk_folder.py:129]
  py311: 343 D install setuptools from wheel /usr/local/lib/python3.11/site-packages/virtualenv/seed/wheels/embed/setuptools-67.4.0-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
  py311: 343 D install wheel from wheel /usr/local/lib/python3.11/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]
  py311: 343 D install pip from wheel /usr/local/lib/python3.11/site-packages/virtualenv/seed/wheels/embed/pip-23.0.1-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
  py311: 347 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/setuptools to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/setuptools [virtualenv/util/path/_sync.py:36]
  py311: 348 D copy /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/pip-23.0.1-py3-none-any/pip-23.0.1.virtualenv to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/pip-23.0.1.virtualenv [virtualenv/util/path/_sync.py:36]
  py311: 348 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/wheel [virtualenv/util/path/_sync.py:36]
  py311: 349 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/pip-23.0.1-py3-none-any/pip-23.0.1.dist-info to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/pip-23.0.1.dist-info [virtualenv/util/path/_sync.py:36]
  py311: 357 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/pip-23.0.1-py3-none-any/pip to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/pip [virtualenv/util/path/_sync.py:36]
  py311: 369 D copy /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.virtualenv to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/wheel-0.38.4.virtualenv [virtualenv/util/path/_sync.py:36]
  py311: 371 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.dist-info to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/wheel-0.38.4.dist-info [virtualenv/util/path/_sync.py:36]
  py311: 381 D generated console scripts wheel3 wheel3.11 wheel wheel-3.11 [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
  py311: 464 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/_distutils_hack to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/_distutils_hack [virtualenv/util/path/_sync.py:36]
  py311: 465 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/pkg_resources to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/pkg_resources [virtualenv/util/path/_sync.py:36]
  py311: 513 D copy directory /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/setuptools-67.4.0.dist-info to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/setuptools-67.4.0.dist-info [virtualenv/util/path/_sync.py:36]
  py311: 517 D copy /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/setuptools-67.4.0.virtualenv to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/setuptools-67.4.0.virtualenv [virtualenv/util/path/_sync.py:36]
  py311: 518 D copy /root/.local/share/virtualenv/wheel/3.11/image/1/CopyPipInstall/setuptools-67.4.0-py3-none-any/distutils-precedence.pth to /builds/<org>/<project>/.tox/py311/lib/python3.11/site-packages/distutils-precedence.pth [virtualenv/util/path/_sync.py:36]
  py311: 519 D generated console scripts  [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
  py311: 559 D generated console scripts pip pip-3.11 pip3 pip3.11 [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
  py311: 559 I add activators for Bash, CShell, Fish, Nushell, PowerShell, Python [virtualenv/run/session.py:61]
  py311: 562 D write /builds/<org>/<project>/.tox/py311/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
  py311: 562 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	version_info = 3.11.2.final.0 [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	virtualenv = 20.21.0 [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
  py311: 562 D 	base-executable = /usr/local/bin/python [virtualenv/create/pyenv_cfg.py:34]
  py311: 563 E internal error [tox/session/cmd/run/single.py:58]
  Traceback (most recent call last):
    File "/usr/local/lib/python3.11/site-packages/tox/session/cmd/run/single.py", line 45, in _evaluate
      tox_env.setup()
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/api.py", line 249, in setup
      self._setup_env()
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/python/runner.py", line 107, in _setup_env
      self._install_deps()
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/python/runner.py", line 111, in _install_deps
      self._install(requirements_file, PythonRun.__name__, "deps")
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/api.py", line 96, in _install
      self.installer.install(arguments, section, of_type)
      ^^^^^^^^^^^^^^
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/python/virtual_env/api.py", line 73, in installer
      self._installer = Pip(self)
                        ^^^^^^^^^
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/python/pip/pip_install.py", line 26, in __init__
      super().__init__(tox_env)
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/installer.py", line 15, in __init__
      self._register_config()
    File "/usr/local/lib/python3.11/site-packages/tox/tox_env/python/pip/pip_install.py", line 29, in _register_config
      self._env.conf.add_config(
    File "/usr/local/lib/python3.11/site-packages/tox/config/sets.py", line 64, in add_config
      raise RuntimeError("config set has been marked final and cannot be extended")
  RuntimeError: config set has been marked final and cannot be extended
    py311: FAIL code 2 (0.32 seconds)
    evaluation failed :( (0.40 seconds)
  ```
  
  ## Minimal example
  
  If possible, provide a minimal reproducer for the issue:
  
  ```console
  [tox]
  envlist = py311
  skipsdist = True
  
  
  [testenv]
  # Expose ENV variables in calling shell to tox
  passenv = PIP_TRUSTED_HOST,PIP_INDEX,PIP_INDEX_URL,PIP_EXTRA_INDEX_URL
  
  install_command =
      pip install {opts} {packages}
  
  deps =
      setuptools==67.3.2
      wheel==0.38.4
      pytest==7.2.1
      pytest-cov==4.0.0
      pytest-sugar==0.9.6
      coverage==7.1.0
  
  commands =
      # Install ourselves
      pip install -e .
  
      # pytest-cov doesn't seem to play nice with -p
      coverage run -p -m pytest -s -p no:warnings --junitxml=report.xml tests
  ```
  