release-haskell.nix: include nixos tests for xmonad
release-haskell.nix: include nixos tests for xmonad

I've recently discovered they were broken by a recent XMonad upgrade,
something we should keep tabs on in the future.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs