Use install instead of cp to copy bootstrap tools

We need to preserve modification times on bootstrap tools, but cp -p
also tries to preserve flags, which fails if OBJROOT is on NFS. A -N
option was added to cp for this purpose, but trying to use that would
break cross-building on hosts that don't have that option. The best
remaining option is install -p, which we already assume is present.

PR: 275030
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D53751