release-lib: add option to use another package set

This allows one to use release-lib for his own package trees, like this (release.nix for Hydra):
```
{ supportedSystems ? [ "x86_64-linux" ] }:
let
  lib = import <nixpkgs/pkgs/top-level/release-lib.nix> {
    inherit supportedSystems;
    packageSet = import <mytree>;
  };
in
lib.mapTestOn (lib.packagesWithMetaPlatform lib.pkgs)
```