fix: don't create duplicate workers when clustering is disabled

What does this PR do? (Please give us a brief description of what this PR does.)

This PR fixes a bug that starts a duplicate instance of each queue worker when run in non-clustered mode (MK_DISABLE_CLUSTERING=1).

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packages/backend/src/boot/entry.ts

@@ -75,7 +75,7 @@ async function main() {
 			ev.mount();
 		}
 	}
-	if (cluster.isWorker || envOption.disableClustering) {
+	if (cluster.isWorker) {
 		await workerMain();
 	}