Update opam workflow to avoid double runs

Author: jmid

.github/workflows/opam.yml

@@ -4,7 +4,12 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
 
 jobs:
   build-and-test: