chore(mac): use mac cp for -R ops

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 97c6331b.