Cache python installs

[merlinz01]

This pull request introduces support for caching Python installs in the GitHub Action, allowing users to cache not only dependencies but also the Python interpreter itself.

This works by setting the UV_PYTHON_INSTALL_DIR to a subdirectory of the dependency cache path so that Python installs are directed there.

Fixes #135

[eifinger]

I would rather not spread this functionality over the code if not necessary and avoid setting env vars.

We can also solve this by making use of the fact that cache.restoreCache and cache.saveCache accept more than one path.

Before running the actions determine if cachePython is true and then determine the pythonDir by capturing the output of uv python dir.

Also add another segment to the cache key to show if cachePython is set.

Finally lets also test this with a test in test.yml. e.g. Setting up the cache, restoring the cache and check if the required files are in the expected dir.

[merlinz01]

Almost ready, just need to figure out why the test is failing.

[eifinger]

I see you are iterating to get the test to pass. Some comments that might help.

[eifinger]

This looks really nice. Great work. Thank you!

Small typo and we are good to go. But already leaving an approval. Will release this tomorrow.