nixos/environment: prepend /etc/xdg to XDG_CONFIG_DIRS

Previously, we were only adding profile-relative paths to XDG_CONFIG_DIRS
variable. That required very ugly hacks like
https://github.com/NixOS/nixpkgs/issues/33282#issuecomment-524550842
to be able to configure XDG stuff.

Now, we are prepending the variable with /etc/xdg, allowing us to
simply use `environment.etc."xdg/…"` options.

nixos/modules/programs/environment.nix

@@ -21,6 +21,7 @@ in
         PAGER = mkDefault "less -R";
         EDITOR = mkDefault "nano";
         XCURSOR_PATH = [ "$HOME/.icons" ];
+        XDG_CONFIG_DIRS = [ "/etc/xdg" ]; # needs to be before profile-relative paths to allow changes through environment.etc
       };
 
     environment.profiles = mkAfter