mark integration_ui_ios as flaky (#23437)

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 0cc7e5a..9d344d0 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -405,6 +405,7 @@
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
     timeout_in_minutes: 20
+    flaky: true
 
   ios_sample_catalog_generator:
     description: >