🚸 use GitHub token by default (#61)

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)).

action.yml

@@ -13,6 +13,7 @@ inputs:
       "Used to increase the rate limit when retrieving versions and downloading
       uv."
     required: false
+    default: ${{ github.token }}
   enable-cache:
     description: "Enable caching of the uv cache"
     default: "false"