fix: shorebird flutter checkout can fail silently on Windows

[bryanoltman]

Description

Shorebird commands on windows are executed through shorebird\bin\shorebird.ps1. This silently performs a checkout of our Flutter fork if needed, which can fail if the paths are too long. We currently redirect this output to null, so the failure is silent. Without redirection, errors like the following are printed:

warning: unable to access 'dev/integration_tests/ios_app_with_extensions/ios/watch Extension/Assets.xcassets/Complication.complicationset/Graphic Circular.imageset/.gitattributes': Filename too long