Add unfree packages support

[juh9870]

Is your feature request related to a problem? Please describe.

Some packages I use are licensed with unfree license. In a regular nixos, there area multiple ways to allow those, but this utility allows none of them.

Describe the solution you'd like

Some setting to allow unfree packages to be evaluated.

Describe alternatives you've considered

I tried setting NIXPKGS_ALLOW_UNFREE=1 and using --impure, I specified { allowUnfree = true; } in ~/.config/nixpkgs/config.nix, neither worked

[aanderse]

i imagine you can use the same workaround i mentioned here