Execute bash from path in she-bang

[timja]

macOS ships with bash 3.x and has no plans to update it.

At least prep.sh uses newer bash syntax.

It was hardcoded to use bin/bash in the she-bang which isn't recommended

I've updated it to look in the user's PATH instead

➜  bom git:(re-test-project-configure-page) ✗ /bin/bash --version
GNU bash, version 3.2.57(1)-release (arm64-apple-darwin21)
Copyright (C) 2007 Free Software Foundation, Inc.
➜  bom git:(re-test-project-configure-page) ✗ bash --version
GNU bash, version 5.1.0(1)-release (x86_64-apple-darwin20.1.0)
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[basil]

NixOS users will be very pleased with this commit. 😄