Support an environment variable override for `nox.options.default_venv_backend`

[edgarrmondragon]

How would this feature be useful?

Often times it's easier to set an environment variable than passing the same argument to multiple commands.

Describe the solution you'd like

A new environment variable that's used as the default value of --default-venv-backend when present.

Some possible names: NOX_DEFAULT_VENV_BACKEND or NOXDEFAULTVENVBACKEND

Describe alternatives you've considered

Something like

# noxfile.py

import os

import nox

nox.options.default_venv_backend = os.environ.get("NOX_DEFAULT_VENV_BACKEND", "virtualenv")

Anything else?

If this accepted, I can start working on a PR :)

[henryiii]

NOX_DEFAULT_VENV_BACKEND sounds good to me.