feature: add a python-path output

[mayeut]

Description:
Expose a python-path output containing the chosen Python executable path.

Related issue:
fix #309
mitigate #132

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[mayeut]

The failure for e2e-cache / Test poetry (Python pypy-3.7-v7.x, windows-latest) seems unrelated to changes in this PR.

[vsafonkin]

Hi @mayeut, why you don't want to use pythonLocation environment variable for this purpose?

[henryiii]

The output works doesn't rely on modifying the environment, and doesn't get overwritten. So you could do multiple setup-python's, and access the correct output. An example of this might be problematic:

- uses: actions/setup-python@v3
  with:
    python-version: "3.7"
- uses: some/composite-action@v1 # Runs a different setup-python internally
# Now pythonLocation is no longer the 3.7 one!

[mayeut]

The failure for e2e-cache / Test poetry (Python pypy-3.7-v7.x, windows-latest) seems unrelated to changes in this PR.
(the previous failure on the same test went away with a relaunch of the workflow by @vsafonkin)