enable manually running CI (once merged to default branch)

.github/workflows/ci.yml

@@ -1,6 +1,7 @@
 # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions
 name: ci
 on:
+  workflow_dispatch:
   push:
   # schedule:
     # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule