ci: reduce nolibdecor build to xwayland only
libdecor is only relevant for Xwayland, so no need to build all the other DDX'es for this build test. Remvoving some unncessary load from CI machines by disabling all DDX'es but Xwayland here.
Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net