Because we don't know if mac BSD cp or GNU cp will be first on the path,
where there is ambiguity (e.g. with -R flag), we will use the macOS cp
command in /bin to ensure that we have consistent results. The
alternative would be to fixup each instance of the cp parameters so that
they work consistently with either cp command, but given these scripts
can run only on macOS, that seems unnecessary.
This replaces the earlier change in 97c6331b5.
Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
