autopkgtest job is broken in backports and experimental when the package to test build-depends on another package from the same repository

In https://debusine.debian.net/debian/developers/work-request/280457/ I made a test upload to trixie-backports of a package with a "Build-Depends: debhelper (>= 13.27~)" that can only be satisfied in trixie-backports. The build correctly picked the build dependency from the repository but the autopkgtest job is failing to setup the testbed due to this: https://debusine.debian.net/debian/developers/artifact/2917771/#L82

@terceiro I think ci.debian.net is running autopkgtest for packages in experimental and experimental might have the same problem from time to time. Is this something that is handled by debci in some way? Are you injecting some pinning information for this?

I wonder what's the correct solution here. Shall we inject some pinning that will systematically disable the effect of the "NotAutomatic" flag for all repositories? Shall we do that only when we detect that a repository has such a flag?

Time spent on this should be tracked under the Sovereign Tech Agency project, work package E.