flake: make lib an option of darwinSystem

[rhoriguchi]

With nixosSystem you do this, so you can add your own custom functions to lib.

let lib = (_: super: { custom = import ./lib.nix { lib = super; }; });
in nixosConfigurations.host1 = lib.nixosSystem { ... }

This allows you to pass lib to darwinSystem so you can use the custom functions in nix-darwin.

[emilazy]

This should already work if you pass lib = … to darwinSystem. Though I don’t recommend it; mismatching lib and pkgs can cause all kinds of headaches. It’s better to use specialArgs to inject your own separate library instead.