I also use NixOS, but if the target was not NixOS, I don't think you should be requesting the set up as needing fixing by the author. It just doesn't sound right - or maybe it is just me. NixOS isn't the defacto standard, and breaks the Linux FHS to achieve all the good stuff it does do.

Either try to package it or use a docker image or maybe raise an issue noting the blocker and request it as a feature for some changes to give an easier path for to having it build more easily for NixOS

Apart from that, as expected, the docker image that is produced following the instructions is working fine with NixOS as host. All it needed for the build was the openssl packaged on the host.