Use arm64 machines for ios_debug_sim_arm64 test. (#44920)

The test is failing when running on a x64 machine and we are pinning this test to use arm64.

Bug: flutter/flutter#132993

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: godofredoc

ci/builders/mac_unopt.json

@@ -232,7 +232,8 @@
             "recipe": "engine_v2/tester_engine",
             "drone_dimensions": [
                 "device_type=none",
-                "os=Mac-12"
+                "os=Mac-12",
+		"cpu=arm64"
             ],
             "gclient_variables": {
                 "download_android_deps": false