Add workflow to clean up PR branch caches on close

.github/workflows/cleanup-caches.yml

@@ -0,0 +1,33 @@
+name: Cleanup Caches by Branch
+
+on:
+  pull_request:
+    types:
+      - closed
+
+permissions:
+  actions: write
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cleanup
+        run: |
+          gh extension install actions/gh-actions-cache
+
+          REPO=${{ github.repository }}
+          BRANCH="refs/pull/${{ github.event.pull_request.number }}/merge"
+          echo "Fetching list of cache keys"
+          cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH | cut -f 1)
+          echo "Deleting caches..."
+          ## Temporarily disable errexit so individual delete failures don't abort the loop.
+          set +e
+          for cacheKey in $cacheKeysForPR
+          do
+              gh actions-cache delete $cacheKey -R $REPO -B $BRANCH --confirm
+          done
+          set -e
+          echo "Done"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}