action: use the most recent python from tool cache to run the GitHub Action

[mayeut]

This allows to drop python 3.6 without losing support for ubuntu-18.04 runners c.f. #508.

Blocked on actions/setup-python#411
Blocked on a new release of actions/setup-python
v4.1 has been released.

[joerick]

LGTM!

[henryiii]

Ahh, v4 doesn't point at v4.1.0 yet. Why is this passing, actually?

[mayeut]

Why is this passing, actually?

Seems that invalid inputs are just warnings... Since Python gets setup properly, there's no reason for the workflow to fail.

Warning: Unexpected input(s) 'update-environment', valid inputs are ['python-version', 'python-version-file', 'cache', 'architecture', 'token', 'cache-dependency-path']

Let's wait for v4 being tagged again.
I'm not sure it's worth adding another workflow to test the action does not update the environment.

[henryiii]

v4 tag has been moved. I've moved the test to 18.04 to verify that works as expected. (though we'd need to drop 3.6 to verify it's working I guess since cibulidwheel works the built-in 3.6 & cibuildwheel doesn't print the host Python version).

[henryiii]

This also should avoid getting the CMake environment variables set pointing at the global Python instance (also added in 4.1.0).

[joerick]

Shall we go back to ubuntu-latest runner before merging?

[henryiii]

Leaving this until after we drop 3.6 might be helpful. Okay either way.

[mayeut]

Let's get this in then.