Allow overriding of environment/configuration variables from the root crate

[mosteo]

Although libraries should not set variables, it makes sense to me to allow the final client of all dependencies, i.e. the root crate, to force override any variable that may have been set (note that environment variables also cause this trouble) in case of conflict.

See e.g. https://github.com/alire-project/alire/issues/1687