feat: add download-from-astral-mirror input (#897)

## Summary

Add a new boolean input `download-from-astral-mirror` (default: `true`)
that controls whether uv is downloaded from the Astral mirror or
directly from GitHub Releases.

When set to `false`, the mirror rewrite is skipped entirely and the
download goes straight to GitHub Releases.

Closes: #870

src/setup-uv.ts

@@ -154,6 +154,7 @@ async function setupUv(
     inputs.checksum,
     inputs.githubToken,
     inputs.manifestFile,
+    inputs.downloadFromAstralMirror,
   );
 
   return {