release/tools: use same pkg settings in containers as in /etc/pkg/FreeBSD.conf

Instructions in /etc/pkg/FreeBSD.conf and elsewhere recommend putting
changes in /usr/local/etc/pkg/repos/FreeBSD.conf so bring OCI containers
into line as well.

Reviewed by: dfr, ivy
Differential Revision: https://reviews.freebsd.org/D54090

MFC after: 5 days
Sponsored by: SkunkWerks, GmbH