Changed to ubuntu in the yaml file

handwritten/bigtable/cloudbuild.yaml

@@ -15,7 +15,7 @@ steps:
   id: 'install-dependencies'
 
 # 2. (Optional) Run pre-system-test.sh if it exists
-- name: 'ubuntu/bash'
+- name: 'ubuntu'
   entrypoint: 'bash'
   args:
   - '-c'
@@ -27,7 +27,7 @@ steps:
     else
       echo "No .kokoro/pre-system-test.sh found, skipping."
     fi
-  dir: '.' # Changed to '.'
+  dir: '.'
   id: 'run-pre-test-hook'
 
 # 3. Configure environment variables for the tests and run system tests