[CP] [devicelab] explicitly enable vulkan validation in test. (flutte…

…r#147382) (flutter#148863)

Fix for release build failure https://ci.chromium.org/ui/p/flutter/builders/prod/Linux_pixel_7pro%20hello_world_impeller/3962/overview

Part of flutter#142659

this test expects validation layers on, so we need to explicitly enable them before we can turn them off by default.

dev/devicelab/bin/tasks/hello_world_impeller.dart

@@ -54,6 +54,7 @@ Future<TaskResult> run() async {
       'run',
       options: <String>[
         '--enable-impeller',
+        '--enable-vulkan-validation',
         '-d',
         device.deviceId,
       ],