give benchmark output dill path (flutter#40857)

Author: Jonah Williams

dev/devicelab/lib/tasks/hot_mode_tests.dart

@@ -23,7 +23,7 @@ TaskFunction createHotModeTest() {
     final File benchmarkFile = file(path.join(_editedFlutterGalleryDir.path, 'hot_benchmark.json'));
     rm(benchmarkFile);
     final List<String> options = <String>[
-      '--hot', '-d', device.deviceId, '--benchmark', '--verbose', '--resident',
+      '--hot', '-d', device.deviceId, '--benchmark', '--verbose', '--resident', '--output-dill', path.join('build', 'app.dill')
     ];
     int hotReloadCount = 0;
     Map<String, dynamic> twoReloadsData;