revert update of schedule

.github/workflows/validate-stacks.yaml

@@ -21,7 +21,7 @@ on:
   pull_request:
     branches: [main]
   schedule:
-    - cron: "*/30 * * * *"
+    - cron: 0 5 * * *
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.number || github.ref }}