CI: Add workflow_dispatch trigger to scheduled repository workflow

By adding the workflow_dispatch trigger, the scheduled workflow can
also be manually triggered if the scheduled run failed to run due to
scheduling issues or because of an error that might have been fixed
since then.

.github/workflows/scheduled.yaml

@@ -1,6 +1,7 @@
 name: Scheduled
 run-name: Scheduled Repository Actions ⏰
 on:
+  workflow_dispatch:
   schedule:
     - cron: 17 0 * * *
 permissions: