sysutils/firstboot-pkgs: Bootstrap and update pkg unconditionally

The background of this patch is available at:
https://lists.freebsd.org/pipermail/freebsd-cloud/2020-April/000234.html

Even a pkg -N success, the following pkg install may still fail because of
the repository version doesn't match between client and server.

Therefore, unconditionally bootstrap and update pkg at firstboot to ensure pkg
and local metadata are update-to-date.

Approved by: cperciva (maintainer)
MFH: 2020Q2
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24517