Default to isolated Python virtual environments (venv)

I tracked down the persistent issue on the autobuilder setup where worker
builds were failing after maintenance specifically on systems like
RHEL8-based hosts where we don't run the system python3.

We had `virtualenv_site_packages: true` configured across the cluster to
better test distro-provided Python packages and stay aligned with current
docs. Setting `virtualenv_site_packages: false` for RHEL8 and OpenSUSE 15.6
based workers resolved the issue.

I propose we stop using site packages on the autobuilder in general to
avoid similar issues in the future. I already have the changes ready for
the next maintenance window.

If we do so our documentation should probably be updated to match so
developers avoid these same environment edge cases. We could
update ref-manual/system-requirements.html to default to standard, isolated
virtual environments and pull host Python dependencies via PyPI rather than
assuming distro packages.

Let me know what you think or if there are any edge cases I haven't
considered.

-- 
Michael Halstead (he/him)
Linux Foundation / Yocto Project
Staff IT Engineer