Merge branch 'fix_nest' into 'release_v0.1_202109'

Fix(pynn-brainscales): Enforce correct log4cxx version

See merge request technical-coordination/devops/platform/ebrains-spack-builds!27

(cherry picked from commit c5211577)

121d8497 Quick-fix to add nest's lib64 site-packages to PYTHONPATH