Configure when uv should install Python

[johnthagen]

How would this feature be useful?

Support for uv installing Python was added in

• feat: support for uv-installed pythons #842

But as discussed in #842 (comment) no configuration option was added to opt-out of this behaviour.

Describe the solution you'd like

As mentioned in #842 (comment) add new configuration options:

--download-python=auto,always,never

Additionally, this should be configurable within a noxfile.py:

import nox

nox.options.download_python = "never"  # type: Literal["auto", "always", "never"]

This feature should also be documented.

[johnthagen]

CC @saucoide