`nh darwin` rollback support

[lmdevv]

darwin-rebuild supports rolling back to the previous generation and switching to a specific generation:

darwin-rebuild --rollback switch
darwin-rebuild --switch-generation <generation> switch

nh os exposes the equivalent functionality through nh os rollback, including the --to option, but nh darwin currently only provides switch, build, and repl.

Could nh darwin get the same interface?

nh darwin rollback
nh darwin rollback --to <generation>