feat: fix duplicate CI runs on pull requests (#513)

Author: mserico

.github/workflows/main.yml

@@ -1,5 +1,9 @@
 name: main
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 jobs:
   test:
     runs-on: ubuntu-latest