Update BrainScaleS packages and py-torch

• This updates BrainScaleS software (packages: hxtorch, pynn-brainscales & oppulance) to BrainScaleS current (stable) HEAD. Amongst other things, this should fix issues with newer py-torch versions ⇒ update py-torch to upstream spack state (0.18.1) and add to spack.yaml; this also requires a newer log4cxx now.
• It fixes a build issue (not finding googletest headers) of hxtorch and pynn-brainscales by removing system paths from the build-related environment variables for the build of those packages. This was an issue on some sites which add system paths via compiler settings (or other means).
• It moves dependencies and package preferences out of spack.yaml (and into the packages or package.yaml, whatever fits better).
• It enables tests for all BrainScaleS packages (only a very basic install test for oppulance).
• It removes the "keep running" try-except blocks in the build env preparation steps of the packages.