Is your feature request related to a problem? Please describe.
Unable to define default configuration for current process context only.
This is especially useful when running on an Azure build agent which might be running multiple pipelines concurrently.

Describe the solution you'd like
Support defining default configuration from environment, as supported by Azure CLI.
Experience should looks like:

1. $ az login
2. $ export AZURE_DEVOPS_EXT_DEFAULTS_ORGANIZATION=https://dev.azure.com/myorg
3. $ az repos list --project myproject --detect false

Additional context
Originally reported as a bug in #1121, but turned out to be a feature request according to latest comment.