CI: Only run CI workflow once per PR

The previous configuration would run CI twice for every PR, on:
- The merge commit with the base branch.
- The tip commit of the PR branch.

The latter is only useful for debugging. However, we still want to run
CI directly on the `main` branch, to ensure that any PRs with runtime
conflicts but not code conflicts get tested in combination.

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: CI checks
 
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    branches: master
 
 jobs:
   test: