Allow passing in package set in standalone mode

[tazjin]

The current preamble in standalone-mode looks like this:

with import <nixpkgs> {};

At work we usually write derivations that can be built standalone
(using just nix-build [-A foo]), but that receive a package set as a
parameter when built as part of a deployment.

An easy way to achieve this that does not break previous behaviour is
letting the preamble look like this instead:

{ pkgs ? import <nixpkgs> {}}: with pkgs;

A fix for this is applied on the devel branch.