Mark flaky tests as flaky (#17004)

dev/devicelab/manifest.yaml

@@ -313,6 +313,7 @@ tasks:
       iOS.
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
+    flaky: true
 
   microbenchmarks_ios:
     description: >
@@ -333,6 +334,7 @@ tasks:
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
     timeout_in_minutes: 20
+    flaky: true
 
   ios_sample_catalog_generator:
     description: >