added a conda_load_cmd, to be used for loading environments (#1)

[kelbling]

On some occasions conda environments need to be loaded using source activate <path_to_conda_env> instead of conda activate <path_to_conda_env>. E.g.: conda/conda#8536
The issue is old and might be solved in most cases but still appears.
This can now be solved by setting the conda_load_cmd in the corresponding enviornment settings, e.g.:

environments:
  example_env:
    type: conda
    packages: [some_package]
    depends: [conda]
    conda_activate_cmd: source

[FussyDuck]

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