Add mamba to options, error message, etc.

[Tolker-KU]

How would this feature be useful?
Mamba was recently added as an alternative to conda (see #448) but is missing in options and error messages. See for example

https://github.com/theacodes/nox/blob/f55e9c53581d78d9373eb2868837036fd598a7a1/nox/_options.py#L319

https://github.com/theacodes/nox/blob/f55e9c53581d78d9373eb2868837036fd598a7a1/nox/sessions.py#L677

Describe the solution you'd like
I would like to have 'mamba' added where meaningful. An approach for fixing this might be searching for 'conda' and adding 'mamba' where appropriate.