CI: consider caching pip downloads in tests.yml

[fariss]

There are five workflows that use the actions/setup-python action and run pip install but don't enable pip caching. Each workflow run downloads all deps from PyPI even when requirements.txt hasn't changed. Especially in tests.yml which spawns 12+ matrix runners per PR, I think we can reduce CI time.

  - uses: actions/setup-python@v5.0.0                                                                                                   
    with:                                                                                                                                 
      python-version: "3.13"                                                                                                            
      cache: 'pip'

[aclfe]

I am on it