[fuchsia] Do not run Fuchsia tests in Linux Fuchsia builder

These are still being run in Linux Fuchsia FEMU. We have identified
that most of the failures are not-flutter related and typically
end up blocking the Flutter tree.

See: https://github.com/flutter/flutter/issues/69336 and child issues
for more context
Change-Id: If297dcb7b5848acc72c8f09383d354e6b8136a40
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/8081
Reviewed-by: Keyong Han <keyonghan@google.com>
Reviewed-by: David Worsham <dworsham@google.com>
Commit-Queue: Kaushik Iska <kaushikiska@google.com>