Enable the sessions to access the original cwd

[DiddiLeija]

Closes #469. Store the original cwd inside global_config, and then enable the sessions to get access to it.

CC @theacodes

[DiddiLeija]

@theacodes I supposed you want me to enable the access to the original cwd as an option for the CLI, right? Or do I have to do something different?

[DiddiLeija]

Tell me before I add more commits, please.

[DiddiLeija]

Oh, I think I should add this as a session property, like @theacodes suggested. Now, I understand that the original cwd will be used like this:

# is this a "session property"?
nox --use-original-cwd

Is that true? Or should I do something else?

[DiddiLeija]

This was superseeded by #472.