podman: add mtree-netbsd package

this tool will make it possible to compare image versions in a manner
which is more granular than "podman diff", specifically, it will allow
us to ignore the file modification times when comparing different builds
of the same image while considering all other important parameters such
as permissions, file types and sizes and content digests

From: https://gitlab.torproject.org/tpo/tpa/base-images/-/commit/d0ff63bf6260ae7ddb9536e383fb5b3638a73284