Adjust Black Sea Recipe to conform to using deepESDL lab

[AliceBalfanz]

When following the recipe for Black Sea, there is a section:

conda create -n blacksea -c conda-forge --file requirements.txt
conda activate blacksea

This is not correct when using deepesdl lab. It is ok to keep this for the case someone wants to do it on their machine, but only as an "alternative" step description. Main solution should be using deep esdl, which would mean to explain how to activate the correct corresponding env in deep-esdl, which would be:

$ conda activate /home/conda/deepesdl/envs/cube-gen