Fix and update brainscales stuff

This fixes two issues:

• The build flow in our rc5 did not support building when PyTorch was not installed in the PYTHONHOME/interpreter site-packages but rather provided via PYTHONPATH — we now look at both, the Python site-packages thingy, but also into PYTHONPATH/sys.path directories.
• In !112 (merged) we also forgot to specify hxtorch's dependency on PyTorch (it's an extension to PyTorch, so it needs headers, and also links against PyTorch).

(→ technically pynn-brainscales does not support a rebuild, but to keep our versions aligned, this also updates the version name for pynn-brainscales to rc6.)