Cache python installs (#621)

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

---------

Co-authored-by: Kevin Stillhammer <kevin.stillhammer@gmail.com>

README.md

@@ -26,6 +26,7 @@ Set up your GitHub Actions workflow with a specific version of [uv](https://docs
     - [Save cache](#save-cache)
   - [Local cache path](#local-cache-path)
   - [Disable cache pruning](#disable-cache-pruning)
+  - [Cache Python installs](#cache-python-installs)
   - [Ignore nothing to cache](#ignore-nothing-to-cache)
   - [GitHub authentication token](#github-authentication-token)
   - [UV_TOOL_DIR](#uv_tool_dir)
@@ -355,6 +356,20 @@ input.
     prune-cache: false
 ```
 
+### Cache Python installs
+
+By default, the Python install dir (`uv python dir` / `UV_PYTHON_INSTALL_DIR`) is not cached,
+for the same reason that the dependency cache is pruned.
+If you want to cache Python installs along with your dependencies, set the `cache-python` input to `true`.
+
+```yaml
+- name: Cache Python installs
+  uses: astral-sh/setup-uv@v6
+  with:
+    enable-cache: true
+    cache-python: true
+```
+
 ### Ignore nothing to cache
 
 By default, the action will fail if caching is enabled but there is nothing to upload (the uv cache directory does not exist).