ci: add workflow_dispatch

Author: Nugine

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
       - main
   schedule: # https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#onschedule
     - cron: '0 0 * * 0' # at midnight of each sunday
-
+  workflow_dispatch:
 
 name: CI