nautilus: fixed mime types

pkgs/desktops/gnome-3/core/nautilus/default.nix

···
       # Thumbnailers
       --prefix XDG_DATA_DIRS : "${gdk_pixbuf}/share"
       --prefix XDG_DATA_DIRS : "${librsvg}/share"
+      --prefix XDG_DATA_DIRS : "${shared_mime_info}/share"
     )
   '';