Make worflow runnable with dispatch

Author: acogoluegnes

.github/workflows/oci-make-nightly.yaml

@@ -5,6 +5,8 @@ on:
     # GitHub advises to schedule jobs NOT at the start of the hour
     # https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule
     - cron: 20 2 * * 1-5
+  workflow_dispatch:
+
 env:
   REGISTRY_IMAGE: pivotalrabbitmq/rabbitmq