Bump @actions/cache to 5.1.0, log cache write denied

src/cache-save.ts

@@ -63,6 +63,10 @@ const cachePackages = async (packageManager: string) => {
 
   const cacheId = await cache.saveCache(cachePaths, primaryKey);
   if (cacheId == -1) {
+    // saveCache returns -1 without throwing when the cache was not saved, e.g.
+    // a reserve collision or a read-only token (fork PR). @actions/cache has
+    // already logged the reason at the appropriate severity, so just trace it.
+    core.debug(`Cache was not saved for the key: ${primaryKey}`);
     return;
   }