tests: fix the missing dependencies for samples gcb (#412)

Author: vishwarajanand

samples/migrations/tests.cloudbuild.yaml

@@ -14,6 +14,11 @@
 
 steps:
   - id: Install dependencies
+    name: python:${_VERSION}
+    entrypoint: pip
+    args: ["install", "--user", "-r", "requirements.txt"]
+
+  - id: Install samples dependencies
     name: python:${_VERSION}
     entrypoint: pip
     dir: samples/migrations