Add support for defining _CONDA_BUILD_ISOLATED_ACTIVATION in meta.yaml

[kenodegard]

Description

The _CONDA_BUILD_ISOLATED_ACTIVATION environment variable is special and can currently only be set prior to running conda build, (e.g. _CONDA_BUILD_ISOLATED_ACTIVATION=1 conda build ...). This makes it hard to enable for packaging teams that do not normally expose this kinds of customizations. So instead of exclusively relying on a pre-set environment variable, let's also allow the value to be set within meta.yml's build/script_env.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[dholth]

Why is the environment variable underscore-prefixed _CONDA_BUILD_ISOLATED_ACTIVATION?

[kenodegard]

To indicate that its private?

[jezdez]

I'm not sure this should be "private" if it becomes part of the recipe format. Are there other ways to add this as a first-level recipe option?

[kenodegard]

@jezdez I don't want this environment variable to become an official meta.yaml setting. IMO isolated mode should be the only mode conda is ever invoked as. We also need to work to eliminate the need to run conda via subprocess in the first place. Furthermore, there needs to be proper API end-points that conda-build should call instead.

[kenodegard]

we are collectively displeased with this solution and should consider something like conda-forge/conda-feedstock#204 instead