feat: add option to disable cache pruning (#134)

This PR adds an input to control whether or not the cache is pruned
before saving.

Closes #122

---------

Co-authored-by: Kevin Stillhammer <kevin.stillhammer@gmail.com>
Co-authored-by: Charlie Marsh <crmarsh416@gmail.com>
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

README.md

@@ -166,6 +166,26 @@ It defaults to `setup-uv-cache` in the `TMP` dir, `D:\a\_temp\uv-tool-dir` on Wi
     cache-local-path: "/path/to/cache"
 ```
 
+### Disable cache pruning
+
+By default, the uv cache is pruned after every run, removing pre-built wheels, but retaining any
+wheels that were built from source. On GitHub-hosted runners, it's typically faster to omit those
+pre-built wheels from the cache (and instead re-download them from the registry on each run).
+However, on self-hosted or local runners, preserving the cache may be more efficient. See
+the[documentation](https://docs.astral.sh/uv/concepts/cache/#caching-in-continuous-integration) for
+more.
+
+If you want to persist the entire cache across runs, disable cache pruning with the `prune-cache`
+input.
+
+```yaml
+- name: Don't prune the cache before saving it
+  uses: astral-sh/setup-uv@v3
+  with:
+    enable-cache: true
+    prune-cache: false
+```
+
 ### GitHub authentication token
 
 This action uses the GitHub API to fetch the uv release artifacts. To avoid hitting the GitHub API