--no-system PACKAGE and --always-prefer-system are incompatible

The --no-system flag from https://github.com/alisw/alibuild/releases/tag/v1.17.17 is a very welcome improvement, but can not be used on slc9 because --always-prefer-system is needed to use system packages.

For development use it would be very useful to have both flags being usable together, with --no-system overriding --always-prefer-system for an individual package (and its dependents)