bsdinstall: Enable FreeBSD-base repo when pkgbase

When performing a pkgbase install, create a configuration file in
/usr/local/etc/pkg/repos/FreeBSD.conf which enables the FreeBSD-base
repository. (This repository is defined in /etc/pkg/FreeBSD.conf as
being disabled by default.)

Reported by: Mark Millard
Reviewed by: markj
MFC after: immediately (needed for 15.0-RC1)
Differential Revision: https://reviews.freebsd.org/D53777

(cherry picked from commit a96230e0329b32cbee0999813787a86eab521da4)