[python] cmakebuilder.py fails when DUNE_SAVE_BUILD is set.

Something was forgotten during the restructuring. When DUNE_SAVE_BUILD=console then code that is supposed to printthe output does not work anymore. I couldn't figure out how to solve this quickly.

https://gitlab.dune-project.org/core/dune-common/-/blob/master/python/dune/generator/cmakebuilder.py#L239 here stdout is not defined.