* Added a command ‘nix-store --print-env $drvpath’ that prints out the

  environment of the given derivation in a format that can be sourced
  by the shell, e.g.

  $ eval "$(nix-store --print-env $(nix-instantiate /etc/nixos/nixpkgs -A pkg))"
  $ NIX_BUILD_TOP=/tmp
  $ source $stdenv/setup

  This is especially useful to reproduce the environment used to build
  a package outside of its builder for development purposes.

  TODO: add a nix-build option to do the above and fetch the
  dependencies of the derivation as well.