pytest `--forked` option is not supported

Summary

Using the --forked option of pytest causes many tests to fail.

Steps to reproduce

Try running:

tox -e py38-nocover -- --forked tests/artifactcache/config.py::test_only_one[storage-project]

This fails, while running:

tox -e py38-nocover -- tests/artifactcache/config.py::test_only_one[storage-project]

Does not fail.

We can see that the test passes fine when run individually, but fails if it is given the problematic --forked option.