Add workflow_dispatch to issue manager workflow

.github/workflows/issue-manager.yml

@@ -1,6 +1,7 @@
 name: Issue Manager
 
 on:
+  workflow_dispatch:
   schedule:
     - cron: "30 10 * * 1" # run at 10:30 every Monday
   # We only use the issue manager for auto-closing, so we only need the cron trigger.