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/download/download-version.ts

@@ -36,6 +36,7 @@ export async function downloadVersion(
   checksum: string | undefined,
   githubToken: string,
   manifestUrl?: string,
+  downloadFromAstralMirror = true,
 ): Promise<{ version: string; cachedToolDir: string }> {
   const artifact = await getArtifact(version, arch, platform, manifestUrl);
 
@@ -52,7 +53,9 @@ export async function downloadVersion(
       ? checksum
       : resolveChecksum(checksum, artifact.checksum);
 
-  const mirrorUrl = rewriteToMirror(artifact.downloadUrl);
+  const mirrorUrl = downloadFromAstralMirror
+    ? rewriteToMirror(artifact.downloadUrl)
+    : undefined;
   const downloadUrl = mirrorUrl ?? artifact.downloadUrl;
 
   try {