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.