treewide: Fix output references to gnome.GConf

nixos/modules/services/x11/window-managers/metacity.nix

···
         start = ''
           env LD_LIBRARY_PATH=${lib.makeLibraryPath [ xorg.libX11 xorg.libXext ]}:/usr/lib/
           # !!! Hack: load the schemas for Metacity.
-          GCONF_CONFIG_SOURCE=xml::~/.gconf ${gnome.GConf}/bin/gconftool-2 \
+          GCONF_CONFIG_SOURCE=xml::~/.gconf ${gnome.GConf.out}/bin/gconftool-2 \
             --makefile-install-rule ${gnome.metacity}/etc/gconf/schemas/*.schemas # */
           ${gnome.metacity}/bin/metacity &
           waitPID=$!