./build-setup.sh should check for existence of the .conda-env before attempting to recreate it

[jerryz123]

Background Work

• Yes, I searched the mailing list
• Yes, I searched the documentation

Feature Description

Users running build-setup.sh when the conda env is already sourced/exists should see an explanatory error message, instead of a conda crash.

Motivating Example

Avoiding user error

[T-K-233]

An alternative would be to let the script always deactivate to base environment before touching the conda env configs?

[jerryz123]

I prefer to error. I don't like scripts which hijack the user's environment.