Describe the bug
G'day, Nox 2021.6.6 seems to fail reusing environments when it finds multiple Python interpreters installed by pyenv. Perhaps this is related to #428 ?

How to reproduce
Install multiple versions with pyenv

$ pyenv install 3.8.9
$ pyenv install 3.9.5
$ pyenv global 3.9.5 3.8.9

This populates ~/.pyenv/shims/ with the following with executable scripts e.g. python, python3.9, python3.8, etc. This folder is typically added to PATH.

Running the following session breaks virtual environment reuse:

@nox.session(python="3.9")
def test(session):
    session.install("pytest", ".")
    session.run("pytest")

Installing/running nox using poetry or a manually-created virtualenv environment in a project seems to resolve this, however.

Expected behavior
Virtual environments are reused in the presence of pyenv-installed Python versions.