gnome.nautilus: Support thumbnailing WebP files

Note that no other GNOME apps currently support WebP on NixOS
because we lack a usable composition method for gdk-pixbuf caches.

pkgs/desktops/gnome/core/nautilus/default.nix

···
 , libexif
 , libseccomp
 , librsvg
+, webp-pixbuf-loader
 , tracker
 , tracker-miners
 , gexiv2
···
       # Thumbnailers
       --prefix XDG_DATA_DIRS : "${gdk-pixbuf}/share"
       --prefix XDG_DATA_DIRS : "${librsvg}/share"
+      --prefix XDG_DATA_DIRS : "${webp-pixbuf-loader}/share"
       --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"
     )
   '';