Refactor `$CONDA_DEFAULT_ENV` into a `context` value

[kenodegard]

rework CONDA_DEFAULT_ENV into a proper condarc value

Originally posted by @kenodegard in https://github.com/conda/conda/issues/12704#issuecomment-1563115656

[github-actions[bot]]

Hi there, thank you for your contribution!

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Thanks!

[kenodegard]

Not stale