Warnings from pip caching steps on CI

[sajith]

Take https://github.com/atlanticwave-sdx/sdx-controller/actions/runs/3595063504, for example. There are repeated warnings about the workflow:

The `set-output` command is deprecated and will be disabled
soon. Please upgrade to using Environment Files. For more
information see:
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

This is from steps where we set up pip cache; there's probably an easier and up-to-date way of setting up pip cache available in actions/setup-python@v4, like so:

- uses: actions/setup-python@v4
  with:
    python-version: '3.9'
    cache: 'pip' # caching pip dependencies