release: move installworld before installkernel

To support -DNO_ROOT work. The top-level installworld target creates a
new METALOG starting with #mtree 2.0 so it needs to be first, to avoid
overwriting installkernel METALOG entries.

Reviewed by: gjb
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29582