More tweaks to the system test script

Change-Id: I9d13ee55682c8dbd7d27de91f540a10a0e9d1792

Author: Jon Wayne Parrott

.kokoro/system_tests.sh

@@ -28,7 +28,9 @@ export GOOGLE_CLIENT_SECRETS=$(pwd)/testing/client-secrets.json
 
 # Run tests
 pip install --upgrade nox-automation
-nox -k "${NOX_SESSION}" || true
+nox -k "${NOX_SESSION}" || ret_code=$?
 
 # Workaround for Kokoro permissions issue: delete secrets
 rm testing/{test-env.sh,client-secrets.json,service-account.json}
+
+exit ${ret_code}