bring back source_repo var in changelog pipeline

.drone.star

@@ -811,7 +811,7 @@ def manifest(ctx):
   }
 
 def changelog(ctx):
-  repo_slug = ctx.build.source if ctx.build.source else ctx.repo.slug
+  repo_slug = ctx.build.source_repo if ctx.build.source_repo else ctx.repo.slug
   return {
     'kind': 'pipeline',
     'type': 'docker',