Update custom-environments.ipynb

[mccabete]

mamba will throw a confusing error if you try to use it to activate your environment. Conda does not.

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[rtapella]

@emileten @wildintellect
are we recommending using conda for everything now, instead of mamba? With the new libmamba solver?

[wildintellect]

@rtapella yes if that's what's installed. Verify with @grallewellyn @anilnatha @sujen1412 that mamba was removed (afaik new workspaces do not have it)

[grallewellyn]

mamba was removed and changes were pushed with the most recent version release.
I already wrote the new instructions to recommend conda instead of mamba and merged into develop of maap-documentation
https://github.com/MAAP-Project/maap-documentation/pull/341/files

Including the change suggested by this PR, so not sure if this PR is still relevant

[emileten]

Yes looks like Grace's work is going to make it to master with this PR to be merged : #345, so we can probably close this !

[rtapella]

closing this as duplicate/redundant