Don't use 'flutter upgrade' on Cirrus (#429)
Don't use 'flutter upgrade' on Cirrus (#429)

This command isn't intended for CI use, and is also slower due to
downloading artifacts that will be immidately discarded.

See flutter/flutter#86037
flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages