Bump @actions/cache to 5.1.0, handle cache write denied (#1026)

src/cache.ts

@@ -146,7 +146,15 @@ export async function save(id: string) {
     return;
   }
   try {
-    await cache.saveCache(packageManager.path, primaryKey);
+    const cacheId = await cache.saveCache(packageManager.path, 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 instead of misreporting that the cache was saved.
+      core.debug(`Cache was not saved for the key: ${primaryKey}`);
+      return;
+    }
     core.info(`Cache saved with the key: ${primaryKey}`);
   } catch (error) {
     const err = error as Error;