Allow user to specify complete build environment

[guyer]

As requested by @ianhbell via email: "Do you think it would be possible to allow the user to specify the entire environment.yml, rather than interleaving the base one with the user-specified one? My current problem is a conflict between sphinx=7.1.2, which does not compile my pdf properly, and sphinx=7.2.6, which does compile, but which conflicts with some package, though I don’t know which one. I am able to build the docs with just my environment on my local machine with python=3.10 so it is something about the combination that is the problem. The conda file in the docker gives python=3.11 and I need python=3.10 to get everybody to play nicely together."