CI: Only run tests on pushes to main

Author: markgoddard

.github/workflows/pull-request.yml

@@ -1,8 +1,10 @@
 ---
 name: Pull request
 on:
-  - push
-  - pull_request
+  push:
+    branches:
+      - main
+  pull_request:
 jobs:
   build:
     runs-on: ubuntu-latest