ci: allow manual execution

https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#workflow_dispatch

.github/workflows/ci.yml

@@ -3,6 +3,7 @@ name: CI
 on:
   push:
   pull_request:
+  workflow_dispatch:
   schedule:
     - cron:  '0 12 * * *' # Every day noon UTC