Environment var HTTP_PROXY is not respected when HTTPTransport is passed as argument

[tscheburaschka]

We are using httpx as the basic HTTP-client in a library of custom API connectors.
We would like to configure the number of retries and the only way we found was to pass a httpx.HTTPTransport object to the instantiation of httpx.Client. However, this unexpectedly stopped the client from respecting the environment variable HTTP_PROXY.
If this is intended behavior, it would benefit from explicit documentation.
However, I would expect the env vars still to work, even when a transport is passed.

nb: AFAIK, the critical lines are here https://github.com/encode/httpx/blob/master/httpx/_client.py#L669