Describe the bug
Running nixos-option nix.nixPath fails because I don't have a nixpkgs entry in my NIX_PATH. This requires me to use my compatibility wrapper (https://github.com/cole-h/nixos-config/tree/c02cdf4702db3e383073426396dea31c40ae6e71/compat) with -I nixpkgs=compat; otherwise, the program aborts.

To Reproduce
Steps to reproduce the behavior:

1. nixos-option nix.nixPath without nixpkgs in NIX_PATH
2. Observe abort due to lack of <nixpkgs/nixos>

Expected behavior
I would expect a --flake flag, just like nixos-rebuild has, to be able to e.g. nixos-option nix.nixPath --flake .#hostname.

Notify maintainers
cc @Ma27 @chkno

Metadata

• system: "x86_64-linux"
• host os: Linux 5.8.7-zen1, NixOS, 20.09.20200912.ac8a276-cosmere (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 3.0pre20200903_8a945d6
• channels(vin): ""
• channels(root): ""
• nixpkgs: /home/vin/.config/nixpkgs/compat

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: installer.tools.nixos-option