Pass env vars to tox

[tw4l]

Description

This enables us to skip youtube-dl tests in GitHub Actions by ensuring that the "CI" env var is passed to tox.

Motivation and Context

Test test_live_video_loader is failing in CI when it should be skipped.

See: https://github.com/webrecorder/pywb/actions/runs/4290394517/jobs/7509706519#step:5:914

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.