Building in write-disabled containerized environments.

[nickjbrowning]

At CSCS we have moved onto using images for providing software stacks. These images are mounted into a read-only directory. This creates an issue with Sphericart, since we attempt to hack the cudnn_version.h file if we are using the PyTorch installation provided by the image.

@Luthaf do you know if we still need this hack or can it be removed? I've commented it out locally on our cluster and it still builds fine with the latest PyTorch.