gnome3.gnome_control_center: fix thumbnailers path

Just like Nautilus (see #29970), GNOME Control Center also uses
gnome-desktop for generating thumbnails. In particular, it tries
to make a thumbnail from a file choosen as a profile picture, and
when it does not succeed, it will not allow that file to be chosen.
Of course, whithout a thumbnailer, it will always fail.

43129a1cfd/panels/user-accounts/um-photo-dialog.c (L190-L192)

Since gnome-desktop scans `thumbnailers` directories under the paths
in `XDG_DATA_DIRS`, gdk-pixbuf  had to be added to the path to provide
access to image thumbnailer.

pkgs/desktops/gnome-3/core/gnome-control-center/default.nix

@@ -41,6 +41,9 @@ stdenv.mkDerivation rec {
   preFixup = with gnome3; ''
     gappsWrapperArgs+=(
       --prefix XDG_DATA_DIRS : "${gnome3.gnome_themes_standard}/share:${sound-theme-freedesktop}/share"
+      # Thumbnailers (for setting user profile pictures)
+      --prefix XDG_DATA_DIRS : "${gdk_pixbuf}/share"
+      --prefix XDG_DATA_DIRS : "${librsvg}/share"
     )
     for i in $out/share/applications/*; do
       substituteInPlace $i --replace "gnome-control-center" "$out/bin/gnome-control-center"