Fix concurrency group for push-only workflow

Author: trask

.github/workflows/build.yml

@@ -11,7 +11,7 @@ permissions:
   contents: read
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  group: ${{ github.workflow }}-${{ github.sha }}
   cancel-in-progress: true
 
 jobs: