[dart/fuzzer] Restrict libFuzzer to linux
[dart/fuzzer] Restrict libFuzzer to linux

Rationale:
Mac and possibly Windows build don't love fuzzing yet.
Change-Id: Ided9734285197e51145102532e1f59550e261bcf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101342
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Commit-Queue: Aart Bik <293afa0533434225484ba3f9bf2859340baba6ea@google.com>

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