[Python] cmake not needed for install but for running scripts

cmake is a python install requirement in the pyproject.toml so the installation of the Python packages works fine. But cmake is not part of the 'python-require' list in dune.module so cmake might not be available when building dune-py.

After adding cmake to the requirements it would be installed into dune-env/bin and then should be available for building dune-env. We could also add ninja and always use that for dune-py.