engine post commit tests are running off of old code

[gaaclarke]

This build was red because of unit tests failures: https://ci.chromium.org/p/flutter/builders/prod/Mac%20iOS%20Engine/6379

This build should have been compiling with the fix: https://ci.chromium.org/p/flutter/builders/prod/Mac%20iOS%20Engine/6380

Despite listing the right SHA in the build, the build didn't actually include the fix and seems to have been running against the previous code since the next build was green.

cc @godofredoc

[gaaclarke]

These tests weren't actually fixed completely, they went from broken to flakey which made them look like there was an infra problem. There is no infra problem.

[github-actions]

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