$ flutter build ios -t lib/main_staging.dart --flavor=staging --release
...

$ flutter install --device-id=nnn
The Xcode project defines schemes: local, prod, staging
You must specify a --flavor option to select one of the available schemes.

$ flutter install --device-id=nnn --flavor=staging
Could not find an option named "flavor".
Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.