Actually, /bin/sh is available on NixOS 🤓

Yes, it's true that #!/usr/bin/env bash is a more reliable way to get bash, but you mentioned that you wanted a POSIX shell, not that you specifically needed bash, so #!/bin/sh will work on any system including NixOS.

Also, export a=b does work in Fish. But you don't have to use it if you don't know how it works.