chore(ci): automatically delete cache entries associated with closed PRs

.github/workflows/cache-cleanup.yml

@@ -0,0 +1,35 @@
+# This workflow cleans up any cache entries associated with a pull request once it has been closed.
+# This prevents us from having many refs/pull/PR_NUMBER/merge cache entries which will never be used.
+#
+# Note that this will affect both PRs being closed with and without being merged.
+
+name: Cleanup closed PR cache entries
+
+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" }}