fix: outer_env issues fixed

[henryiii]

Fix #868. Two issues:

• Setting include_outer_env=False does nothing at all if env= is not passed. The outer env is still included.
• Required environment variables set by nox itself, like VIRTUAL_ENV (required for uv), and CONDA_PREFIX (required for conda-family) would not be included if you do set both.

Fixing this was a bit intrusive: now .env only is the local environment
settings, and removals are added to it using None. The type has changed to match .command, with the None possibility. And then the final environment is built in the _run, where it can correctly process things like adding the bin dir and filtering the None's.

[DiddiLeija]

This looks fine to me, as it seems to fix the issue properly!
However for this one I'd like to hear what other maintainers think ;)

[henryiii]

However for this one I'd like to hear what other maintainers think

Not sure about that, it's been open for nearly three months. ;)

[DiddiLeija]

Yup, that's true, I was just a little concerned about some spots, but I've re-checked and everything is ok!
To me, you're free to merge :)

[henryiii]

Thanks!