Avoid LD_LIBRARY_PATH in pynn-brainscales&hxtorch

We encountered build fails on some HPC sites due to openssl mismatches
between bare-metal tools and spack — especially curl and similar tools
for fetching.
Our build tool, waf, needs to find spack-provided libraries, e.g.
libclang.so, at runtime… so we will only set the environment variable
for this.