Mark smoke_catalina_start_up_ios as flaky (#61737)

https://github.com/flutter/flutter/issues/61736

dev/devicelab/manifest.yaml

@@ -650,6 +650,7 @@ tasks:
       A smoke test that runs on macOS Catalina, which is a clone of the Gallery startup latency test.
     stage: devicelab_ios
     required_agent_capabilities: ["mac-catalina/ios"]
+    flaky: true
 
   smoke_catalina_hot_mode_dev_cycle_ios__benchmark:
     description: >