ci: run on master pushes and cancel superseded runs

.github/workflows/ci.yml

@@ -3,6 +3,12 @@ name: CI
 on:
   pull_request:
     types: [opened, synchronize, reopened]
+  push:
+    branches: [master]
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 
 jobs:
   check: