I had a Theia instance where I installed buildout with pip install -e --user /path/to/checkout and later deleted that /path/to/checkout and this made it impossible to run instance buildout, failing with import error when importing zc.buildout in buildout script.

This is because python load user site packages by default. I believe we don't want this, a broken user site package should not prevent slapos from running buildout.

These changes are about running buildout with PYTHONNOUSERSITE set, so that python ignores user site packages when running buildout.

references:

• https://docs.python.org/3/using/cmdline.html#envvar-PYTHONNOUSERSITE
• https://docs.python.org/3/library/site.html#site.ENABLE_USER_SITE