Make set list check ignore $DESTDIR/usr/pkg?

[Taylor R Campbell <campbell+netbsd-tech-toolchain%NetBSD.org@localhost>]

Is there a way to make the set list check ignore $DESTDIR/usr/pkg when
running build.sh distribution?

I would like to use that as LOCALBASE for pkgsrc cross-compilation, in
order to take advantage of the base installation that's already in
$DESTDIR, but it's annoying to have to move $DESTDIR/usr/pkg out of
the way each time I redo build.sh distribution to appease the set list
checks.

There's a line in src/etc/mtree/NetBSD.dist.base which looks like it
morally ought to have an effect like this, but it doesn't:

./usr/pkg                       ignore optional

I assume the ignore/optional part is for mtree(8) at run-time, not at
build-time.  Maybe there's something else that can be done for the set
lists?

I know I can dig into the scripts to figure this out, but perhaps
someone already knows them well enough to know how to make this work.