buildbot: don't skip building i386 packages

[Mojca Miklavec]

Hi,

A while ago I requested changing mpbb-list-subports to exclude ports
that have already been built and uploaded.

May we please change this to only exclude "native" builds?

Rationale: I would like to force build wine every now and then just
for the sake of building all the universal dependencies. Yes, I know
that we need a better solution for that, but it would be nice to have
at least one workaround to keep the universal binaries of
dependencies up to date.

Mojca