Mark the test as flaky again (flutter#68636)

It seems that I was lucky to see 10 consecutive successes. Now the test
seems to be quite flaky again.

Author: liyuqian

dev/devicelab/manifest.yaml

@@ -235,6 +235,7 @@ tasks:
       with SkSL shader warm-up with e2e.
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios32"]
+    flaky: true
 
   backdrop_filter_perf__timeline_summary:
     description: >