fix: Shorebird CI doesn't take into account that Dart packages can be part of workspace that needs Flutter

[spydon]

Description

When Shorebird CI is on a step with a Dart package that doesn't have a dependency on Flutter it will only install Dart on the runner, but if the Dart package is part of a pub workspace that has Flutter in it it will fail to pub get.
See: https://console.shorebird.dev/ci/checks/53948050564

Expected Behavior

The checks should run fine.

Additional Context

This happens for this package:
https://github.com/flame-engine/flame/tree/main/packages/flame_behavior_tree/behavior_tree