[test matrix] Add back --compiler=dartkb
[test matrix] Add back --compiler=dartkb

This was removed by https://dart-review.googlesource.com/c/sdk/+/69970
but the --compiler flag isn't being picked up automatically. See e.g.:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8937641495482850608/+/steps/vm_tests/0/stdout

Change-Id: Ia0bd7d708c8e7f8ada536915b9147423b37bc4cf
Reviewed-on: https://dart-review.googlesource.com/70783
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>
Commit-Queue: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk