…me if it exists

Python distributors with custom default installation scheme can set a
scheme that can't be used to expand the paths in a venv.
This can happen if build itself is not installed in a venv.
The distributors are encouraged to set a "venv" scheme to be used for this.
See https://bugs.python.org/issue45413
and pypa/virtualenv#2208

Since Python that ships with the macOS developer tools does not have the "venv"
scheme yet, we keep the special case below. Once it gains the "venv" scheme,
it will be preferred.

Fixes pypa#433

Co-authored-by: layday <layday@protonmail.com>