fix exitcode of borg serve, fixes #2910

(cherry picked from commit 583de3eeb1)

src/borg/archiver.py

@@ -231,12 +231,13 @@ class Archiver:
 
     def do_serve(self, args):
         """Start in server mode. This command is usually not used manually."""
-        return RepositoryServer(
+        RepositoryServer(
             restrict_to_paths=args.restrict_to_paths,
             restrict_to_repositories=args.restrict_to_repositories,
             append_only=args.append_only,
             storage_quota=args.storage_quota,
         ).serve()
+        return EXIT_SUCCESS
 
     @with_repository(create=True, exclusive=True, manifest=False)
     def do_init(self, args, repository):