nix: use final.hyprland instead of prev.hyprland in waybar-hyprland

Currently, waybar-hyprland package adds `prev.hyprland` to `$PATH`. This is nixpkgs' default hyprland, not hyprland injected by this overlay. I'd expect we want waybar-hyprland to depend on what's in `pkgs.hyprland` after this (and possibly user's other overlays) are applied – which is `final.hyprland`.