!/nix/store/v7kh3h7xfwjz4hgffg3gwrfzjff9bw9d-bash-4.2-p24/bin/bash

export prefix=dirname "$(cd ${0%/*}/.. && echo $PWD/${0##*/})"

Submitted by Eelco Visser on 21 December 2012 at 12:14

I looked at the source code and it is not pointing to nix store itself, it uses the #!@bash@ which at config should be replaced with the bash it is using, apperently it is using the nix one.
So this could be becuase it is somewhere hardcoded to use the nix one, or it finds the nix one first because of the config of your system.