Use ESMValTool recipes from conda environment if available

[bouweandela]

Description

Use the ESMValTool recipes from the conda environment if available. This should solve #468.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[bouweandela]

@lewisjared What do you think of this approach? Would it work on a Mac? It feels a bit hacky, but it seems to work. Alternatively, it may be possible to add an option to fetch the required recipes using the ref datasets fetch-data command. Or we leave it as is and document that users can specify the directory where to store the recipes using the environment variable:

climate-ref/packages/climate-ref-esmvaltool/src/climate_ref_esmvaltool/recipe.py

Line 156 in 01eabb5

env="REF_METRICS_ESMVALTOOL_DATA_DIR",

, but that may not be user friendly.

[lewisjared]

I'd lean towards an explicit fetch command. That seems easier to documents via a callout for HPC users to check the config and run a command. Everyone else gets the normal path of pooch fetching/retrieving from cache at run time. The recipies may (potentially) not always be in the conda env