Add channel information to mamba installation

[kurianbenoy]

• Since now, setup-conda.sh uses miniconda. In the default channel mamba is not present:

So we have to install mamba this way

conda install -yq mamba -c conda-forge

• Do let me know if I missed anything, and is there anything different now. If yes, I will like to update this article as well:

https://kurianbenoy.com/posts/2022/2022-05-28-fastai-walthrus1.html