Mark flutter_attach_test flaky (#18801)

Introduced in d248725e, but has been red
since landing. Marking flaky rather than reverting since it's the only
failure.

dev/devicelab/manifest.yaml

@@ -274,6 +274,7 @@ tasks:
       Tests the `flutter attach` command.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
+    flaky: true
 
   # iOS on-device tests