Release channel should respect nnbd opt-out

[Awjin]

The official docs recommend setting the sdk's lower bound to opt out of nnbd:

You can opt out of null safety and preserve your code's previous behavior by setting the lower bound of the SDK constraint in your pubspec to 2.11.0 or earlier

I would expect the setup action to also respect the pubspec. However, it seems to just pull the latest sdk, regardless of the lower bound. This is breaking our CI.

[Awjin]

Looks like this is actually an error in the Dart SDK's behavior.