After #109889 I expected Mac hostonly tests to run on either arm or x64 depending on capacity. However the cpu dimension is still set to x86

Example:

When it ran it still has "cpu": "x86" set:
https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20dart_plugin_registry_test/7264/overview

And the swarming task only shows x64 bots as able to run it:
https://chromium-swarm.appspot.com/task?id=5d7adb0db77c1a10&o=true&w=true
https://chromium-swarm.appspot.com/botlist?c=id&c=task&c=cpu&c=os&c=status&d=asc&f=pool%3Aluci.flutter.prod&f=os%3AMac-12&f=cpu%3Ax86&f=device_type%3Anone&s=id

Is there a cpu default set in infra somewhere?

@keyonghan