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

dist/save-cache/index.cjs

@@ -62990,6 +62990,7 @@ function loadInputs() {
   const pythonDir = getUvPythonDir();
   const githubToken = getInput("github-token");
   const manifestFile = getManifestFile();
+  const downloadFromAstralMirror = getInput("download-from-astral-mirror") === "true";
   const addProblemMatchers = getInput("add-problem-matchers") === "true";
   const resolutionStrategy = getResolutionStrategy();
   return {
@@ -63000,6 +63001,7 @@ function loadInputs() {
     cachePython,
     cacheSuffix,
     checksum,
+    downloadFromAstralMirror,
     enableCache,
     githubToken,
     ignoreEmptyWorkdir,