Avoid mutating cached room aliases. (#15038)

This might cause incorrect data in other callers which
are not expecting the canonical alias to be added into
the response.

changelog.d/15038.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where the room aliases returned could be corrupted.

synapse/handlers/directory.py

@@ -485,7 +485,8 @@ class DirectoryHandler:
                 )
             )
             if canonical_alias:
-                room_aliases.append(canonical_alias)
+                # Ensure we do not mutate room_aliases.
+                room_aliases = room_aliases + [canonical_alias]
 
             if not self.config.roomdirectory.is_publishing_room_allowed(
                 user_id, room_id, room_aliases