[linkchecker] add workflow_dispatch

Author: mmcky

.github/workflows/linkcheck.yml

@@ -3,6 +3,7 @@ on:
   schedule:
     # UTC 23:00 is early morning in Australia (9am)
     - cron:  '0 23 * * *'
+  workflow_dispatch:
 jobs:
   link-checking:
     name: Link Checking