When use this interpreter to run buildout, buildout will pre-compile eggs with
different level of optimisations, for this it spawns sys.executable with -O
option (see [1]). Since pymain does not support -O argument and python only
support changing optimization level at startup, we can not easily handle -O
argument, execpt by re-executing ourselves with PYTHONOPTIMIZE environment
variable, which has same effect than setting -O flag

1: https://lab.nexedi.com/nexedi/slapos.buildout/blob/488b2dfd3088cba7b322827c904fe1f93fba73e6/src/zc/buildout/easy_install.py#L1716-1722