Add support for forky

In one of my repositories, I'd like to be able to run lintian from forky (preferably not unstable, for the sorts of reasons given in #532), because the version in trixie doesn't like us having dropped the Priority field from the Source stanza in debian/control. The first step is for such an image to exist.

I notice that trixie support was added in !426 (merged) well before the release of trixie, so it seems worth trying to do so for forky now.