fix(ci): verify deployable cloud artifacts independently (#13192)

Verify source, build the cloud image, and wait for exact-source migrator packages concurrently. Emit Cloud deployable v1 only when every prerequisite succeeds for the merged full SHA.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/docker-cloud.yml

@@ -1,8 +1,6 @@
 name: Docker cloud
 
 on:
-  push:
-    branches: [master]
   workflow_dispatch:
   workflow_call: