top-level: fix nix-shell eval w/nixUnstable
top-level: fix nix-shell eval w/nixUnstable

For a full description of the underlying issue please read
https://github.com/NixOS/nix/issues/4003

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