conda: set channel via keyword

[henryiii]

How would this feature be useful?
Minor addition for a common situation.

Describe the solution you'd like

What about having a channel= keyword argument for conda_install so that one could write channel="conda-forge" if selecting a channel? It would be a bit more natural than "--channel=conda-forge", I think, and likely is very common.

It is possible to set multiple channels - this is required for bioconda, for example. This could be an iterable, channel=["conda-forge", "bioconda"]. Maybe the keyword should be "channels" and always takes a list?

Describe alternatives you've considered

Leave as is.

[FollowTheProcess]

I think this is a good idea, I use conda a little bit and do sometimes run into difficulties with different channels. Having the flexibility to choose per session would be ideal and agree on the --channel inside the function being less natural.

I think this would be fairly easy to implement too as all you'd have to do (I think) is implement the argument here: https://github.com/theacodes/nox/blob/9c696e2bc2e47ec0f2acd322c7e4be941dab0518/nox/sessions.py#L342-L346

Then it effectively just shells out to conda here:
https://github.com/theacodes/nox/blob/9c696e2bc2e47ec0f2acd322c7e4be941dab0518/nox/sessions.py#L403-L412

So as long as conda supports the -c flag to set the channel then this could be easily done