powershell: Unable to use as a login shell

[AmandaCameron]

Issue description

Attempting to add unstable.powershell to environment.shells or users.extraUsers.<username>.shell results in the following error:

error: /nix/store/4km9l5rdin6iz57g3p229v92jbl1cg5w-powershell-6.0.4 is not a shell package

Steps to reproduce

Add the following to your configuration.nix:

let 
 unstable = import <unstable> {};

{
 environment.systemPackages = [ unstable.powershell ];

 environment.shells = [ unstable.powershell ];

 users.extraUsers.test = {
  isNormalUser = true;
  uid = 1000;
  shell = unstable.powershell;
 };
}

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.67, NixOS, 18.03.133174.edd63e05d1e (Impala)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.0.4`
 - channels(root): `"nixos-18.03.133174.edd63e05d1e, unstable"`
 - channels(amandac): `"nixpkgs-18.09pre150352.7c826371c49"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

[jtojnar]

passhtru.shellPath would need to be defined for powershell, see, for example, fish:

nixpkgs/pkgs/shells/fish/default.nix

Lines 172 to 174 in 03b89e3

	passthru = {
	shellPath = "/bin/fish";
	};