Cancel old builds if new commits appear

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

.github/workflows/test.yml

@@ -2,6 +2,10 @@ name: Tests
 
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   test:
     runs-on: ubuntu-latest