set test image cloudbuild directory

Author: BenTheElder

test/images/cloudbuild.yaml

@@ -10,6 +10,7 @@ options:
 steps:
   - name: 'gcr.io/k8s-testimages/gcb-docker-gcloud:v20190906-745fed4'
     entrypoint: make
+    dir: ./test/images/
     env:
     - DOCKER_CLI_EXPERIMENTAL=enabled
     - TAG=$_GIT_TAG