CI: non-remote GH Actions now use a python venv

[gante]

What does this PR do?

This PR changes the cached python environment in non-remote GH actions to fresh virtual environments, that are rebuilt whenever setup.py gets updated. This ensures our test environment is closer to what a new user would see in the wild, and that we don't have package constraints due to other installed libraries (that come bundled in ubuntu-latest).

As a side effect, because the cached venv requires no installation of python packages on a cache hit, we also got slightly faster CI:

• >3 mins faster in Add new model like template tests
• >3 mins faster in Model templates runner
  (total of ~7 mins per full CI run when there is a cache hit)

Examples of runs:
cache miss -> create new venv: https://github.com/huggingface/transformers/runs/6027339356?check_suite_focus=true
cache hit -> load cached venv: https://github.com/huggingface/transformers/runs/6028381114?check_suite_focus=true

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[gante]

(cc @ydshieh )

[sgugger]

LGTM, thanks a lot for diving into this!

[LysandreJik]

LGTM, thanks @gante!