Make nix shell lock to a <nixpkgs> version

[Fresheyeball]

Just a suggestion. But sometimes I use the --shell option like so cabal2nix ./. --shell > shell.nix and it works great. Then I run nix-shell for that project months later, and it fails (since the nixpkgs version has changed). It would be nice if nixpkgs ? import <nixpkgs> {} was made more robust to specify the version of <nixpkgs> it's currently using. Something like:

{ nixpkgs ? import (builtins.fetchTarBall "github.com/NixOS/nixpkgs/archive/<the version>.tag.gz") {}, ...