userchecks: add an xdg_current_desktop check

ref https://github.com/hyprwm/xdg-desktop-portal-hyprland/issues/251

if the XDG_CURRENT_DESKTOP is externally managed (e.g. DE, DM, etc) Hyprland will not overwrite it. In those cases, if that's undesired, portals and other apps depending on it might break.