Clean pkg cache in release media

Recently wifi-firmware-kmod-release pkgs were added in the release
installation media, but unfortunately the pkgs were not cleaned up after
the installation. These kept the stale pkgs in the /var/cache. The pkgs
should be removed with pkg clean.

PR: 283112
Reported by: yanhui.he@broadcom.com
Reviewed by: cperciva, emaste
Approved by: cperciva, emaste, re (implicit)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47942