This is a known for long impurity of nixpkgs glibc; it calls /bin/sh.
With ludo, we came up with the suggestion that glibc could be patched to refer to a statically-linked bash 'sh', for any /bin/sh reference. That shouldn't introduce any dependency to early bootstrap-tools or so, and fix some portability problems.
Some people write 'make' recipes using bash, too, and maybe this change would fix 'make' recipes as well. An example of such trouble happens with 'attr-2.4.46', which fails to build if /bin/sh is dash.
As a note, the attr problem doesn't get solved by the solution proposed by Raskin at #233 , as I've tested.
Eelco today implemented NixOS/nix#24 , so with this, chroot builds can be made to include /bin/sh pointing to bash for builds, without touching the system /bin.
Ludo opened a nix-dev thread about this: http://firstname.lastname@example.org/msg08851.html
IIRC @edolstra is currently not in favor of any patching to system other than possibly making it use PATH instead of directly calling /bin/sh, so closing this for now.