Added manual trigger for workflow

Workflow file only triggers if it resides in the branch. If an old PR based on the mailu repo without CI.yml is tested, then the workflow run will not trigger. The merged commit on TESTING/STAGING branch does not contain the required CI.yml workflow file after all. In these cases simply run the workflow manually on the TESTING or STAGING branch,

.github/workflows/CI.yml

@@ -1,5 +1,10 @@
 name: CI
 on:
+#NOTE: The workflow will ONLY trigger when the branch actually contains the CI.yml workflow file.
+#So if a PR is tested on STAGING/TESTING branch by BORS without this file present, then the workflow
+#will NOT trigger. For these situations, manually start the workflow. This should be resolved once all
+#old PRs without the CI.yml workflow file have been merged.
+  workflow_dispatch:
   push:
     branches:
       - staging