DTD should be disabled for internal bazel workspaces #5020

DTD isn't available for internal builds and is currently triggering this error:

[DartToolingDaemon] [Error] Could not find /usr/lib/google-dartlang/bin/snapshots/dart_tooling_daemon.dart.snapshot. Have you built the full Dart SDK?

We currently assume all SDKs over a certain version have DTD available but this isn't the case here. We should disable DTD for Bazel workspaces.