Make sure to delete the image placements in the manifest builder cleanup code

data/registry_model/test/test_manifestbuilder.py

@@ -67,6 +67,9 @@ def test_build_manifest(layers, fake_session, registry_model):
   assert tag
   assert tag in builder.committed_tags
 
+  # Mark the builder as done.
+  builder.done()
+
   # Verify the legacy image for the tag.
   found = registry_model.get_repo_tag(repository_ref, 'somenewtag', include_legacy_image=True)
   assert found