Where "never" means:
- with and without usedevelop
- with all options of this plugin
Where "always" means:
- with and without usedevelop
- without options of this plugin
Virtualenv 20.0 follows symlinks when populating sys.executable,
so we can't use it. (AFAIUI it's done to better handle cases where the
system executable is changed after a virtualenv is created.)
See: https://github.com/pypa/virtualenv/pull/1522
sys.exec_prefix still points to inside the virtualenv, so use that
in the tests to determine what's being run.
