backend: admin opt-out createrepo after build-deleting
    
    For projects like @python/python3.8, createrepo_c run takes too
    long (up to 1 minute per one chroot).  Deleting large amount of
    builds then creates long action queue which blocks even other
    projects.
    
    We should make actions parallel to mitigate those problems, but
    this is temporary work-around that allows us to "blacklist"
    expensive projects.