Add all needed paths to PYTHONPATH on all architectures
On some systems like Centos7 64 bits, python recipes that produce native libraries are not installed in /lib/pythonx.x/site-packages but in /lib64/pythonx.x/site-packages.
This is correctly tracked by the py_prefix in configuration and the FileProvider correctly lists python files from PY_CAT category.
But when entering cerbero shell, the environment variable PYTHONPATH only includes packages from /lib/pythonx.x/site-packages.
This patch checks if py_prefix points at a different path and eventually adds it to the PYTHONPATH environment variable.