Document how to build `mops` on CPU only despite the presence of a CUDA compiler

[frostedoyster]

This might be as easy as doing CMAKE_MOPS_CUDA=OFF CMAKE_MOPS_TORCH_CUDA=OFF pip install ... (and making sure the torch wheels are CPU-only), but this needs to be documented.

[Luthaf]

Are you sure that CMAKE_XXX in the env is used to set the XXX cmake option? This happens for native options, but I'm not sure it's the case in general.