But even still you are mixing languages, so the problem remains. The issue is not that Nix uses Bash, but that Nix uses different languages for the host-side and build-side. AFAIK people aren't writing the build-side code in the Nix language, and I'm not sure if it even has the features necessary to do it. Guix unifies the different layers of code execution with a single language, and using a Lisp enabled that design.

For a complete explanation of this design, see the "Code Staging in GNU Guix" paper: https://arxiv.org/abs/1709.00833