CI setup

.github/workflows/build.yml

@@ -1,4 +1,16 @@
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - '**'
+  pull_request:
+
+# cancel older runs of a pull request;
+# this will not cancel anything for normal git pushes
+concurrency:
+  group: cancel-old-pr-runs-${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
 
 jobs:
   build: