Workflow to rebuild reverse build-dependencies (ratt)

A simple workflow to rebuild reverse build-dependencies would be useful test transitions in Debusine.

It shouldn't be that hard to have a simple native ratt workflow, based on the same design as the reverse-autopkgtest workflow. I'll file a blueprint PR with a design.

There's an alternative approach of coordinating such a test from the outside of debusine (as https://salsa.debian.org/stefanor/debusine-rebuilds does). This is way more complex for the user, but it has some advantages:

• We can analyze results locally without needing UI in debusine.
• We can file bugs.
• We can confirm regressions by test-building the pristine source.

I suspect we should do both. Build in a simple ratt workflow, and iterate on more complex tools outside debusine.