gtk3: move gtk-update-icon-cache to the main output

This is basically what aa0fa19373 was for gtk2 and Xfce.
Fixes #20874, though I haven't tested it directly.

pkgs/development/libraries/gtk+/3.x.nix

@@ -63,6 +63,8 @@ stdenv.mkDerivation rec {
   postInstall = optionalString (!stdenv.isDarwin) ''
     substituteInPlace "$out/lib/gtk-3.0/3.0.0/printbackends/libprintbackend-cups.la" \
       --replace '-L${gmp.dev}/lib' '-L${gmp.out}/lib'
+    # The updater is needed for nixos env and it's tiny.
+    moveToOutput bin/gtk-update-icon-cache "$out"
   '';
 
   meta = with stdenv.lib; {