nixos/xdg: add gtkUsePortal option to portals

Prior to this change GTK_USE_PORTAL was unconditionally
set to "1". For this to not break things you have to have some
sort of portal implementation in extraPortals.

Setting GTK_USE_PORTAL in this manner is actually only useful
when using portals for applications outside flatpak. For example
people using non-flatpak Firefox who want native filechoosers.
It's also WIP for electron applications to support this.