workflows/periodic-merge: allow manual dispatch

Currently there is an eval fix on master and waiting five hours for it
to propagate into staging-next and staging is annoyingly long.

.github/workflows/periodic-merge-24h.yml

@@ -13,6 +13,7 @@ on:
     # * is a special character in YAML so you have to quote this string
     # Merge every 24 hours
     - cron:  '0 0 * * *'
+  workflow_dispatch:
 
 permissions:
   contents: read

.github/workflows/periodic-merge-6h.yml

@@ -13,6 +13,7 @@ on:
     # * is a special character in YAML so you have to quote this string
     # Merge every 6 hours
     - cron:  '0 */6 * * *'
+  workflow_dispatch:
 
 permissions:
   contents: read