actions: allow manual actions from GitHub UI

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

.github/workflows/build.yml

@@ -5,6 +5,7 @@ on:
   pull_request:
   schedule:
     - cron: '37 22 * * *'
+  workflow_dispatch:
 
 permissions:
   contents: read