Upgrading in fact may be challenging - that Win 7 box is a shared build machine with quite conservative update policy. I can try setting up a clone locally, but it will take some time.
Let’s try to approach it from a different perspective: is there a way to tell EBuild which runtime to load (or run EBuild against a specific runtime)? It might be that we can get away with copying .NET 4.7 to some local directory and running EBuild against it.