Don't run duplicate actions when pushing to a branch in the main repo

.github/workflows/ci.yml

@@ -1,7 +1,11 @@
 ---
 name: ci
 
-on: [pull_request, push]
+on:
+  pull_request:
+  push:
+    branches:
+      - master
 
 env:
   # Just a reassurance to mitigate sudden network connection problems