ci: Run gallery iOS transition perf build test on arm64 bots

[cbracken]

This moves Mac_build_test flutter_gallery__transition_perf_e2e_ios in .ci.yaml from cpu=x86tocpu=arm64`, as part of a transition of all our mac builds/tests to arm64 hosts.

This doesn't rename the test since, despite the "perf" in its name, it's not skia-perf impacting. We build on a new machine type, but the tests/measurements are unchanged, and are still running on the same class of iOS device as before.

Manually verified via:

• Checked config/flutter_config.star in flutter/infra; the cpu=x86 restriction only applies to the %sMac Devicelab Test Drone pool, not to app compilation build tests (devicelab_drone_build_test).
• Flutter Gallery iOS project has compiled without problem on arm64 hosts arm64 mac hosts for eons and already separately in CI (Mac_arm64_ios flutter_gallery_ios__compile).

No test changes since there are no code changes; building and passing existing tests is the test.

Issue: #189144

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[gemini-code-assist]

Code Review

This pull request updates the CPU architecture from x86 to arm64 in the drone_dimensions for the flutter_gallery__transition_perf_e2e_ios target within .ci.yaml. There are no review comments, and I have no feedback to provide.

[jmagman]

I'm not clear why this one was ever pinned to a x64 bot...
#111164

This is the only mac_build_test which was part of an aborted build+test model #103542.

If you are only looking at this test, instead of changing letting the builder override cpu you can remove the dimension and instead set it in mac_build_test.

However, I don't think there's any reason for mac_build_test to exist anymore, it can probably be renamed Mac_ios flutter_gallery__transition_perf_e2e_ios (which involves creating the new one in bringup and then removing bringup and removing the old test once it passes in presubmit). And then remove mac_build_test at the top.