Change trigger of Step 0 workflow (#137)

`create` -> `push` to `main`

.github/workflows/0-welcome.yml

@@ -6,8 +6,10 @@ name: Step 0, Welcome
 # This will run every time we create push a commit to `main`.
 # Reference: https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows
 on:
-  create:
   workflow_dispatch:
+  push:
+    branches:
+      - main
 
 # Reference: https://docs.github.com/en/actions/security-guides/automatic-token-authentication
 permissions: