Mamba on ARM-based macOS

I haven't looked into this in detail, but looking at [this StackOverflow](https://stackoverflow.com/questions/71515117/how-to-set-up-a-conda-osx-64-environment-on-arm-mac) and some issues users have had in the past installing software, I wonder if we should add to instructions: 

`conda config --env --set subdir osx-64`
