ci: avoid duplicate CI runs

* run CI on push to master or synchronize PR
* cancel in-progress workflows for updated PR

.github/workflows/tests.yml

@@ -1,6 +1,18 @@
 name: Tests
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - 'master'
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+    branches:
+      - 'master'
+
+# Cancel any in-progress CI runs for a PR if it is updated
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  cancel-in-progress: true
 
 jobs:
   check_compilation_unix_like: