Add URL and hash to nix-shell for downloading 21.05

nixpkgs-2105.json

+{ "name": "stable2105"
+, "url": "https://releases.nixos.org/nixos/21.05/nixos-21.05.804.5de44c15758/nixexprs.tar.xz"
+, "sha256": "002zvc16hyrbs0icx1qj255c9dqjpdxx4bhhfjndlj3kwn40by0m"
+}

shell.nix

 let
-  nixpkgs-pin = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
-  nixpkgs-src = builtins.fetchTarball nixpkgs-pin;
-  nixpkgs = import nixpkgs-src { };
-  stable2105 = import (builtins.fetchTarball https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz) { };
+  nixpkgs = import (builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./nixpkgs.json))) { };
+  stable2105 = import (builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./nixpkgs-2105.json))) { };
 in
 { pkgs ? nixpkgs }:
 pkgs.mkShell {