Update makefile.yml to run on pushes to main branch only

.github/workflows/makefile.yml

@@ -4,8 +4,9 @@ on:
   # Run using manual triggers from GitHub UI:
   # https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow
   workflow_dispatch: {}
-  # Run on pushes:
-  push: {}
+  # Run on pushes to the main branch only:
+  push:
+    branches: [ "main" ]
   # Run on every pull request:
   pull_request: {}