Create a GH workflow to cleanup the GH cache

The GitHub actions cache is helpful to speed up CI jobs. Once a branch
is closed, we don't need the cacche anymore so we should delete it so
the disk space can be reclaimed.

This workflow was copied from https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries.

.github/workflows/cleanup-cache.yml

@@ -0,0 +1,42 @@
+name: Cleanup Cache
+
+# for testing
+on: [pull_request]
+
+# on:
+#   pull_request:
+#     types:
+#       - closed
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+        
+      - name: Cleanup
+        run: |
+          gh extension install actions/gh-actions-cache
+          
+          REPO=${{ github.repository }}
+          BRANCH="refs/pull/${{ github.event.pull_request.number }}/merge"
+
+          while true
+          do
+            cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH -L 100 | cut -f 1 )
+
+            if [ -z "$cacheKeysForPR" ]
+            then
+              break
+            fi
+
+            ## Setting this to not fail the workflow while deleting cache keys. 
+            set +e
+            for cacheKey in $cacheKeysForPR
+            do
+              gh actions-cache delete $cacheKey -R $REPO -B $BRANCH --confirm
+            done
+          done
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}