chore: add dir field to run-tests-and-delete-vm step in cloudbuild

Author: zhixiangli

packages/google-cloud-storage/cloudbuild/zb-system-tests-cloudbuild.yaml

@@ -49,6 +49,7 @@ steps:
   # The VM is deleted after tests are run, regardless of success.
   - name: "gcr.io/google.com/cloudsdktool/cloud-sdk"
     id: "run-tests-and-delete-vm"
+    dir: "packages/google-cloud-storage"
     entrypoint: "bash"
     args:
       - "-c"