nanobsd: Add _xxx_pkg_add_var_db_files_to_metalog

[senguptaangshuman17_gmail.com]

Summary

Add /var/db/pkg files to NANO_METALOG since pkg generates them dynamically
and does not emit them into package manifests.

[jlduran]

I'm confused with the commit message: "since pkg generates them dynamically and does not emit them into package manifests."

1. Dynamic generation is intrinsic to pkg.
2. Perhaps most importantly is why we are doing this. It is because pkg does not generate a valid mtree (METALOG). All these metalog-related _xxx_ issues should point to https://github.com/freebsd/pkg/issues/2672.