Missing interpreters now error the session on CI by default

[FollowTheProcess]

Closes #340

This PR adds a check to determine if nox is running on CI when evaluating sessions with missing interpreters so that such sessions fail by default when being run on CI providers.

This has the desired effect:

Skipped locally showing CI warning

CI=true causes an error

Note non-zero exit code

--error-on-missing-interpreters for reference

I have some questions I'd like people's input on:

• Should this be documented or just rely on the warning message?
• The warning message is kinda long, any other suggestions that get the point across? Maybe just say it will fail in CI and leave it there without mentioning --error-on-missing-interpreters? Given that it will show any time a session is skipped a long message could get annoying.
• Currently we just check for CI existing in os.environ should it explicitly check for true?

[DiddiLeija]

Should this be documented or just rely on the warning message?

I think some documentation is OK. Maybe a little note at https://nox.thea.codes/en/latest/tutorial.html#testing-against-different-and-multiple-pythons?

Maybe just say it will fail in CI and leave it there without mentioning --error-on-missing-interpreters?

If that's the case, I would prefer some documentation, to say what this warning won't say?

[DiddiLeija]

Looks good for me now :)

[FollowTheProcess]

@henryiii Does this work for you? Since #340 had quite a lot of discussion around this I'd like to get lots of eyes on this to make sure everyone is happy 🙂

[cjolowicz]

Can we make it so CI determines just the default for the option?

This has two advantages IMO:

1. There is a single source of truth for the setting.
2. You can override the default in CI using --no-error-on-missing-interpreters. IOW overriding works both ways.

[FollowTheProcess]

Can we make it so CI determines just the default for the option?

Sure, that's a good idea, let me play around some more and get back to you 👍🏻

[FollowTheProcess]

Huh, thought I had it working... standby 😂

[FollowTheProcess]

Okay, think I've found a nice way of doing this. Behaviour now:

Skips show warning message

CI turns warning into an error

Can override the error with --no-error-on-missing-interpreters

[cjolowicz]

Thanks! Some minor suggestions inline.

[FollowTheProcess]

Cool! Thanks for the review, should be good to go now if everyone's happy 🙂