Improve environment variable handling in Cogent Code

[kkoreilly]

What about just running all commands through separate Goal processes in Cogent Code, which would bypass the environment variable issue entirely since Goal would get the right environment variables on startup? We will be replacing a lot of the current command infrastructure with a Goal terminal interface soon anyway.

Originally posted by @kkoreilly in https://github.com/cogentcore/cogent/pull/355#discussion_r1816839955