virt-manager: add missing gsettings schema

Without this it'll complain and abort when clicking "Take Screenshot" or
"Browse Local" when creating a new VM and looking for an CD-ROM image to boot
from:

GLib-GIO-ERROR **: Settings schema 'org.gtk.Settings.FileChooser' is not installed

pkgs/applications/virtualization/virt-manager/default.nix

@@ -52,7 +52,7 @@ buildPythonPackage rec {
             --prefix GIO_EXTRA_MODULES : "${dconf}/lib/gio/modules" \
             --prefix GSETTINGS_SCHEMA_DIR : $out/share/glib-2.0/schemas \
             --prefix LD_LIBRARY_PATH : ${gtk3}/lib/:${libvirt-glib}/lib/:${vte}/lib:${gtkvnc}/lib${optionalString spiceSupport ":${spice_gtk}/lib"} \
-            --prefix XDG_DATA_DIRS : "$out/share:${gsettings_desktop_schemas}/share:${gtk3}/share:\$XDG_DATA_DIRS"
+            --prefix XDG_DATA_DIRS : "$out/share:${gsettings_desktop_schemas}/share:${gtk3}/share:$GSETTINGS_SCHEMAS_PATH:\$XDG_DATA_DIRS"
     done
 
     ${glib}/bin/glib-compile-schemas "$out"/share/glib-2.0/schemas