test: keep existing PYTHONPATH in test_noxfile_script_mode - #1190

test_noxfile_script_mode now prepends the fake nox-999 package to PYTHONPATH instead of replacing it.

Fixes #1189

When nox is importable only through PYTHONPATH, as in FreeBSD ports where the staged install is exposed that way during tests, the replaced value drops nox and the subprocess fails with No module named nox.

The failure only shows up when the whole file runs, because earlier in-process tests leave the working directory at a noxfile directory (via os.chdir in nox/tasks.py), so the source tree is no longer on sys.path either.

The fake nox-999 metadata stays first on the path, so the test still covers the #1157 behavior.

This is a test-only change and nox/_cli.py is untouched.

To reproduce the ports setup, I made a venv with only the runtime dependencies plus pytest and staged nox with uv pip install --no-deps --target /tmp/stage ..

Before this change:

$ PYTHONPATH=/tmp/stage python -m pytest tests/test_main.py -o addopts=
E       AssertionError: assert 1 == 0
...
/tmp/nox1189-venv/bin/python: No module named nox
FAILED tests/test_main.py::test_noxfile_script_mode - AssertionError: assert ...
1 failed, 130 passed, 2 skipped

After this change:

$ PYTHONPATH=/tmp/stage python -m pytest tests/test_main.py -o addopts=
131 passed, 2 skipped

The normal session still passes:

$ nox -s tests-3.12 -- tests/test_main.py -k script_mode
4 passed

nox -s lint is clean as well.