Use the temporary solution described here:
https://github.com/fluttercommunity/flutter_workmanager/issues/551

Instead of adding the Package normally like this:

workmanager: ^0.5.2

Add it with a specific commit like this:

  workmanager:
    git:
      url: https://github.com/fluttercommunity/flutter_workmanager.git
      ref: b783000