debug: remote controller inspects environment variables of server instead of client

[jedevc]

The remote controller environment handling logic is scattered, and currently, many environment variables will be found in the server's environment instead of the client's.

Related:

• https://github.com/docker/buildx/pull/1581 is a similar issue relating to local paths from the client
• https://github.com/docker/buildx/pull/1614 should start to move some of the handling into the client, however, there are still some environment variables in the build package that we should extract and handle before invoking the controller.