Autopkgtest image is not cleaned for non-default branches

In feature branches, the autopkgtest image is built with STABLE release but cleaned along with other images with RELEASE variable.

In the container registry, the image tag appears as bookworm_<feature-branch> (stable), whereas we attempt to clean sid_<feature-branch> (release)

Same applies for arm64v8/autopkgtest

Correct me if I am mistaken. Thanks!