run tests on pushes to master only, and all pull requests

Author: danarnold

.github/workflows/ci.yml

@@ -1,5 +1,9 @@
 name: CI
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 jobs:
   tests:
     name: Ruby ${{ matrix.ruby }} • ${{ matrix.gemfile_name }}