virtualenv inside LCG distribution

Tests with LCG_96b (thanks @swertz for the reports) showed a few isues, summarizing:

• venv creates a pip executable (small wrapper) that's not compatible with the pip module that's actually picked up, so one should use python -m pip instead (see also https://github.com/pypa/pip/issues/5599)
• building the library that depends on libtorch in !20 (merged) fails because a different version of the torch package is in the distribution than the latest one (which I tested with there)
• from studying the above: the LCG setup script adds the site packages to PYTHONPATH, which overrides what the virtualenv relies on (such that it's impossible to pick up a more recent package from the virtualenv). Setting the PYTHONHOME to /cvmfs/sft.cern.ch/lcg/views/LCG_96bpython3/x86_64-centos7-gcc8-opt/lib/python3.6, removing some parts of the PYTHONPATH and creating the venv with --system-site-packages seems to swap the order as expected, but doesn't fix the original issue with libtorch yet