text: Title: tox 4: passenv can no longer use factors
  Body: My tox.ini files are affected by a change to the parsing of `passenv` in tox 4: 
  I can no longer use factors to conditionalize which environment variables are passed:
  ```
  passenv =
      sagepython, sagewheels:  SAGE_VENV
      sagewheels:              SAGE_SPKG_WHEELS
  ```
  (from https://github.com/sagemath/sage/blob/develop/pkgs/sagemath-categories/tox.ini#L29)
  
  _Originally posted by @mkoeppe in https://github.com/tox-dev/tox/discussions/2699#discussioncomment-4403536_
      