CI: Run pkgbase METALOG lint script

tools/pkgbase/metalog_reader.lua checks for errors in METALOG (for
pkgbase staging), such as hard links with differing modes, duplicate
entries, etc. Run it as part of the Cirrus-CI job to prevent
regressions.

Reviewed by: manu, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37521