ci: update global workflows (#661)

.github/workflows/pull-requests.yml

@@ -3,12 +3,18 @@
 # Don't make changes to this file in this repo as they will be overwritten with changes made to the same file in
 # the above-mentioned repo.
 
+# Ensure PRs are made against `nightly` branch.
+
 name: Pull Requests
 
 on:
   pull_request_target:
     types: [opened, synchronize, edited, reopened]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   check-pull-request:
     name: Check Pull Request