Bulk builds do not run on NetBSD 9.2

[nia <nia%NetBSD.org@localhost>]

Hi,

due to the PKGDB changes bulk builds do not run on a fresh NetBSD 9.2
chroot, instead failing with the error:

Compatibility pkgdb location exists, but PKG_DBDIR not specified.
This may cause unexpected issues. To avoid problems, add
PKG_DBDIR=/var/db/pkg to /etc/mk.conf.

I'm not really sure what the unexpected issues are or whether it
matters where the package database is on a build server, so I've
just set it to use the legacy location in mk.conf. I noticed the
TNF builders on nyftp do the same.

Does anyone know what the unexpected issues are and why this
message is required?