change shebangs from /bin/bash to /usr/bin/env bash

These changes allow all these scripts to be executed on Unix-like OS which are otherwise POSIX compliant but do not include a /bin/bash (instead having bash in another place by default). I have successfully built ACL2S with these changes on NixOS (following https://github.com/michzappa/nix-acl2s-build). I'm not a /user/bin/env expert so it could likely use a run on a "standard" Linux distro too, but I don't see why it wouldn't work.

Thanks to Jake Chvatal for the suggestion.