Allow-unfree switch

[jonringer]

With the new checkMeta it will also mark unfree packages "as broken" since it refuses to evaluate them now.

I know for some users that don't want unfree software on their machines, this is probably a big issue. And the current filtering of unfree packages is probably preferable.

We should probably allow for some way to pass --allow-unfree, so that users that do want to check all packages can.

Noticed this in: NixOS/nixpkgs#99264