feat: add workflow to clear gh cache on pr close (#1897)

.github/workflows/clear_cache.yml

@@ -0,0 +1,35 @@
+# GitHub Caches are branch specific with non-default branches having access
+# to the `default` branch.
+# It appears that you cannot configure https://github.com/actions/cache to
+# only cache if `default` branch.
+# This workflow helps with housekeeping the caches.
+# See https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries
+name: Post-merge Cache Cleanup
+on:
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cleanup
+        run: |
+          gh extension install actions/gh-actions-cache
+
+          echo "Fetching list of cache key"
+          cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH -L 100 | cut -f 1 )
+
+          ## Setting this to not fail the workflow while deleting cache keys.
+          set +e
+          echo "Deleting caches..."
+          for cacheKey in $cacheKeysForPR
+          do
+              gh actions-cache delete $cacheKey -R $REPO -B $BRANCH --confirm
+          done
+          echo "Done"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          BRANCH: refs/pull/${{ github.event.pull_request.number }}/merge