Previously calling release-lib for only x86_64-darwin like:

  import ./release-lib.nix {
    supportedSystems = [ "x86_64-darwin" ];
  }

would still cause the packageSet to be evaluated for
x86_64-linux. This is annoying if you are evaluating this on
x86_64-darwin and your packageSet uses Import From Derivation to
produce a jobset. In that case you need to have a x86_64-linux build
machine configured in order to finish the evaluation.

This commit initializes `pkgs` for the currentSystem such that all IFD
derivations are build for the current system.

This shouldnt change anything for hydra.nixos.org since there
`builtins.currentSystem == "x86-64-linux"`.

(cherry picked from commit 76034ed502f8c8859e45c44725dfe290bf90821a)