I export pkgsrc over NFS from a macOS host to a variety of VMs, each of
which sets ${PACKAGES} to a subdir of ${PKGSRCDIR}/packages (on the same
NFS mount) so that the generated packages all land in one place.

>From my Debian, Devuan, Ubuntu, and CentOS 8 (but not 7) VMs,
copying ${STAGE_PKGFILE} to ${PKGFILE} fails to preserve permissions,
which fails the copy and the "package" target with it. The error
looks like so:

===> Building binary package for foo-4.2
=> Creating binary package /home/schmonz/trees/pkgsrc-cvs/packages/Debian/All/foo-4.2.tgz
/bin/cp: preserving permissions for ‘/home/schmonz/trees/pkgsrc-cvs/packages/Debian/All/foo-4.2.tgz’: Operation not permitted

Assuming permissions on built packages are merely a nice-to-have, avoid
failing if we couldn't preserve them. Patch from sjmulder@, for whom
this fixes a similar problem with FreeBSD with ZFS.

(schmonz)