cos/tools: Add default zone

BUG=None
TEST=presubmit, cloudbuild test
RELEASE_NOTE=None

Change-Id: I10203025306951992d6d6429006c76e807c38008
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/66350
Tested-by: Nandhini Rengaraj <nrengaraj@google.com>
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Shuo Yang <gshuoy@google.com>
diff --git a/testing/gpu_test/gpu_test.yaml b/testing/gpu_test/gpu_test.yaml
index e2a84a1..3723690 100644
--- a/testing/gpu_test/gpu_test.yaml
+++ b/testing/gpu_test/gpu_test.yaml
@@ -18,7 +18,7 @@
   '_INPUT_PROJECT': 'cos-cloud'
   '_DRIVER_VERSION': ''
   '_DEPS_DIR': ''
-  '_ZONE': ''
+  '_ZONE': 'us-central1-a'
 steps:
 - name: 'gcr.io/cloud-builders/docker'
   args: ['build', '-f', 'src/cmd/cos_customizer/Dockerfile', '-t', 'cos_customizer', '.']