nix-repl> pkgs.config
{ allowUnfree = true; }

nix-repl> pkgs.cudatoolkit
error: Package ‘cudatoolkit-9.2.148.1’ in /nix/store/kzd44wh814pdrb7ickivrvlf1jdj48yl-nixpkgs-unstable/pkgs/development/compilers/cudatoolkit/default.nix:187 has an unfree license (‘unfree’), refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.

«derivation