Avoid duplicate CI runs triggered by pushes to pull requests (#2178)

https://github.com/charliermarsh/ruff/pull/2157#discussion_r1087179996

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

.github/workflows/ci.yaml

@@ -1,6 +1,10 @@
 name: CI
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref_name }}-${{ github.event.pull_request.number || github.sha }}