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

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

If this change needs to override an active code freeze, provide a comment explaining why. The code freeze workflow can be overridden by code reviewers. See pinned issues for any active code freezes with guidance.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[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.

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.

For more guidance, visit Writing a golden file test for package:flutter.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.