ci(frontend): Limit concurrency of frontend workflow on PRs (#38889)

Author: mattgauntseo-sentry

.github/workflows/frontend.yml

@@ -6,6 +6,12 @@ on:
       - master
   pull_request:
 
+# Cancel in progress workflows on pull_requests.
+# https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 # hack for https://github.com/actions/cache/issues/810#issuecomment-1222550359
 env:
   SEGMENT_DOWNLOAD_TIMEOUT_MIN: 3