chore: Use pull_request.head.ref for concurrency group

Author: effigies

.github/workflows/pixi-lock.yml

@@ -2,7 +2,7 @@ on:
   pull_request_target
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.head.ref }}
   cancel-in-progress: true
 
 jobs: