CI: allow manual CodeQL runs via workflow_dispatch

Author: AndreyVMarkelov

.github/workflows/codeql.yml

@@ -9,6 +9,7 @@ on:
       - main
   schedule:
     - cron: 0 0 * * 1
+  workflow_dispatch:  # allow on-demand manual scans
 
 permissions:  # least privilege
   contents: read