update fails for lxqt qt5

[rardiol]

Issue description

downloading Nix expressions from ‘https://d3g5gsiof5omrk.cloudfront.net/nixos/unstable/nixos-17.09pre102441.45344fd/nixexprs.tar.xz’...
downloading ‘https://d3g5gsiof5omrk.cloudfront.net/nixos/unstable/nixos-17.09pre102441.45344fd/nixexprs.tar.xz’... [9071/9121 KiB, 187.6 KiB/s]
unpacking channels...
building Nix...
building the system configuration...
error: undefined variable ‘qt5’ at /nix/store/k888yfl7a8abcy9y6ac1qswl8wzaq6lj-nixos-17.09pre102441.45344fd/nixos/pkgs/desktops/lxqt/default.nix:74:7
(use ‘--show-trace’ to show detailed location information)

I assume it has something to do with this?

055c0f4#diff-b868212404d6c3cfbef4b767b494f70eL76

Steps to reproduce

sudo nixos-rebuild switch --upgrade

Technical details

• System: 17.03pre100896.4450327 (Gorilla)
• Nix version: nix-env (Nix) 1.11.6
• Nixpkgs version: "17.09pre102441.45344fd"