pipeline: identify tree objects during build

Every time a stage has been successfully built, the contents of
the tree now corresponds to the stage and can thus be identified
via the id of the stage.
When the tree is being written to, i.e. on consecutive attempts
of stage builds, the `id` of the tree object will automatically
be reset.

osbuild/pipeline.py

@@ -249,6 +249,10 @@ class Pipeline:
                 results["success"] = False
                 return results, None, None
 
+            # The content of the tree now corresponds to the stage that
+            # was build and this can can be identified via the id of it
+            tree.id = stage.id
+
             if stage.checkpoint:
                 object_store.commit(tree, stage.id)