[native_assets_builder] test/test_data/transformer_test.dart flaky

[dcharkes]

It seems that our in-hook caching solution is broken on the MacOS bot:

❌ test/test_data/transformer_test.dart: transformer caching (failed)

  FINE: 2024-12-12 11:43:40.569175: Try `dart pub outdated` for more information.
  Expected: a string containing 'Transformed 1 files.', 'Reused 9 cached files.' in order
    Actual: 'Transformed 3 files.\n'
              'Reused 7 cached files.\n'
              ''
  
  package:matcher                             expect
  test/test_data/transformer_test.dart 124:7  main.<fn>.<fn>
  ===== asynchronous gap ===========================
  test/helpers.dart 46:5                      inTempDir

Error: 51 tests passed, 1 failed.
Error: Process completed with exit code 1.

https://github.com/dart-lang/native/actions/runs/12295543113/job/34313026197?pr=1809

It's flaky, different amounts of files are cached:

https://github.com/dart-lang/native/actions/runs/12295543113/job/34312641116?pr=1809

I suspect it's because I removed the 1 second sleeps in #1750, but this hook still uses time-stamp hashing.