[devicelab] Mark flutter_gallery_ios32__start_up as flaky. (#62646)

The test device is causing failure of this test, we will need a couple days to physically access the device and fix the issue. So mark it as flaky to unblock the tree.

Bug: https://github.com/flutter/flutter/issues/62633

dev/devicelab/manifest.yaml

@@ -493,6 +493,7 @@ tasks:
       Measures the startup time of the Flutter Gallery app on 32-bit iOS (iPhone 4S).
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios32"]
+    flaky: true # https://github.com/flutter/flutter/issues/62633
 
   flutter_gallery_ios32__transition_perf:
     description: >