Makefile: Don't allow install{world,kernel} with pkgbase

Using these targets on a pkgbase system will cause the installed system
to become out of sync with the package database, which is almost certain
to cause issues the next time pkg(8) is used.

To prevent users doing this accidentally, disallow install* if we detect
that FreeBSD-runtime is installed in the target. The check can be
overridden with DESTDIR=/ for users who are sure they want to do this.

MFC after: 3 days
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52879