Fix Catalina hot reload test by specifying the platform is iOS. (#38708)

dev/devicelab/bin/tasks/smoke_catalina_hot_mode_dev_cycle__benchmark.dart

@@ -6,7 +6,9 @@ import 'dart:async';
 
 import 'package:flutter_devicelab/tasks/hot_mode_tests.dart';
 import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/framework/adb.dart';
 
 Future<void> main() async {
+  deviceOperatingSystem = DeviceOperatingSystem.ios;
   await task(createHotModeTest());
 }