Fix: Restore flake module to default modules list
The merge accidentally removed the flake module from the default modules list. This restores it to match the 0.18.0 ChangeLog which says flake validation is enabled by default.
Without this fix, users who don't explicitly import the flake module will see errors like:
error: The option 'perSystem.x86_64-linux.gitlab.ci.flake' does not exist
This is a critical fix for 0.18.0.