oksh: add shellPath for use as user shells
oksh: add shellPath for use as user shells

This allows the user shell to be set to pkgs.oksh instead of needing
a string like "${pkgs.oksh}/bin/oksh".

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs