beinstall: Avoid chrooting into new world

The new world may use system calls that are not in the currently-running
kernel, so we cannot chroot into the new environment to run `make
installworld`, etcupdate, etc. Partially revert commit 16702050ac95
("beinstall: perform pre-installworld steps") and switch back to using
DESTDIR for installworld and so on.

Reported by: olivier
Reviewed by: olivier
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50682