We should aim for a solution with sane defaults that work as most users
would expect it.
The current default leads to a cache that is never invalidated and thus
never updated.
This also mirrors the default behaviors for setup-python which use
`**/requirements.txt`, `**/Pipfile.lock` or `**/poetry.lock`
Also make it clear that the uv cache on the filesystem can be uploaded to the GitHub Actions cache, but they are distinct things.
Also make it clear that the local cache path can be changed without using `enable-cache: true`
This PR adjusts the action so that it is no longer necessary to
explicitly specify the GitHub token to circumvent the API rate limit.
To this end, it simply passes the repository's GitHub token (available
via `${{ github.token }}`) as a default for the `github_token` action
input.
This is similar to how this is handled in, e.g.,
https://github.com/peter-evans/create-pull-request; specifically
[here](5354f85616/action.yml (L4-L6)).
