Run mac unopt arm builds with arm toolchain (flutter/engine#41353)

Add `--force-mac-arm64` to arm gn commands running on Mac ARM bots to compile natively and not with Rosetta.  Sometimes we've seen random slow-downs in CI when lots of Rosetta processes run at the same time.

This will disable goma for these builds but it doesn't seem to be much slower:

`build ios_debug_sim_arm64` step on prod:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20Production%20Engine%20Drone/123894/overview - 1.1 minute
vs this PR:
https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/747210/overview - 1.8 minute

engine/src/flutter/ci/builders/mac_unopt.json

@@ -141,6 +141,7 @@
                "--unoptimized",
                "--no-lto",
                "--prebuilt-dart-sdk",
+               "--force-mac-arm64",
                "--mac-cpu",
                "arm64"
             ],
@@ -187,6 +188,7 @@
                 "debug",
                 "--simulator",
                 "--no-lto",
+                "--force-mac-arm64",
                 "--simulator-cpu",
                 "arm64"
             ],