update workflow trigger criteria

.github/workflows/build-test.yml

@@ -1,6 +1,12 @@
 name: Build & Test
 
-on: [pull_request, push, workflow_dispatch]
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    # The branches below must be a subset of the branches above
+    branches: [ "main" ]
+  workflow_dispatch:
 
 permissions:
   contents: read