sbuild workflow binNMU with dep-wait

User story

(This is a follow-up to #89 (closed).)

People scheduling binary-only NMUs via debusine need a way to specify an extra dep-wait constraint when scheduling a binNMU, so that rebuilds against a new dependency wait for that dependency to be built on all architectures.

Error handling

It's possible that the dependency might never become available. The handling in that case should be the same as the handling for any other unsatisfiable build-dependency.

Implementation plan

I think the easiest way to do this would be to pass it through to the --add-depends option to sbuild, which would mean that it behaves the same way as any other sbuild task that dep-waits and needs to be retried later.