fix: in generate_block_key, get the usage key's block_id, not usage_id

UsageKeys don't have a usage_id field. This line would have crashed
if run.

openedx/core/djangoapps/content_libraries/tasks.py

@@ -102,7 +102,7 @@ def generate_block_key(source_key, dest_parent_key):
             branch='library'
         )
         derived_block_key = derive_key(
-            source=source_key_as_v1_course_key.make_usage_key(source_key.block_type, source_key.usage_id),
+            source=source_key_as_v1_course_key.make_usage_key(source_key.block_type, source_key.block_id),
             dest_parent=BlockKey(dest_parent_key.block_type, dest_parent_key.block_id),
         )
         return dest_parent_key.context_key.make_usage_key(*derived_block_key)