[99] Builds from the leaves?

Eventually we want to get Nix's scheduler entirely out of the picture: - Start building from the leaves (rather than from "direct" targets). A "leave" in our context is a .drv for which no extra inputs have to be built. - Create temporary roots for the dependencies and keep them until it has no unbuilt reverse dependencies left - Cleanup (shouldn't be necessary unless evanix was interrupted) (very low priority)