Regression: Nox crashes with AttributeError: module 'sys' has no attribute 'real_prefix' when reusing virtualenvs

[cjolowicz]

Describe the bug
On the main branch, nox -r crashes with AttributeError: module 'sys' has no attribute 'real_prefix'. This appears to be a regression from #418.

How to reproduce
Install Nox into a virtualenv, and run its own test suite using -r.

$ git clone https://github.com/theacodes/nox
$ cd nox
$ virtualenv venv
$ . venv/bin/activate
$ pip install -e .
$ nox -rs tests -p 3.9
nox > Running session tests-3.9
nox > python3.9 -c import sys; print(sys.prefix)
nox > .../nox/.nox/tests-3-9/bin/python -c import sys; print(sys.real_prefix)
nox > Command .../nox/.nox/tests-3-9/bin/python -c import sys; print(sys.real_prefix) failed with exit code 1:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: module 'sys' has no attribute 'real_prefix'
nox > Session tests-3.9 failed.

Expected behavior
Nox reuses the virtual environment without crashing.

[cjolowicz]

[OT, will open a separate issue]

The following line from #418 does not work on Windows:

os.path.join(self.location, "bin", "python"),

This directory is named "Scripts" on Windows.