tox.ini: passenv should be comma-separated, not space-separated

[jelly]

Describe the changes

[jelle@⬢ python311][~/projects/pyfakefs]%HOME=1 USERPROFILE=2 tox -e py311
py311: failed with pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'HOME USERPROFILE'
  py311: FAIL code 1 (0.00 seconds)
  evaluation failed :( (0.03 seconds)

See: tox-dev/tox#2658

Tasks

• Unit tests added that reproduce the issue or prove feature is working
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing
• For documentation changes: The Read the Docs preview builds and looks as expected

[mrbean-bremen]

Thanks!