bsdinstall: Use pkgbase release URL for BETA/RC

Approved by: re (cperciva)
PR: 290238
Reviewed by: ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53214

(cherry picked from commit 604b6458f3f327206a56da7ba80c2df7965a0761)