PYTHONPATH leaks into "envreport"

[f3flight]

Pretty trivial bug.
Even though tox discards PYTHONPATH for pip install, it does not discard it for pip freeze, causing garbage output if PYTHONPATH has any extra packages (i.e. "pip freeze" will see packages from PYTHONPATH, which it should not do).

https://github.com/tox-dev/tox/blob/3.27.0/src/tox/venv.py#L843

Fix is trivial - fetch env and apply ensure_pip_os_environ_ok the same way it is done for pip install, pass env to pcall. I will make a PR.