migrations: ignore maildir when in folder layout

Otherwise we'd be tryhing to move the maildir into itself and error out.

migrations/nixos-mailserver-migration-03.py

@@ -53,6 +53,8 @@ def is_maildir_related(path: Path, layout: FolderLayout) -> bool:
     if layout is FolderLayout.Default and path.name.startswith("."):
         return True
     if layout is FolderLayout.Folder:
+        if path.name in ["mail"]:
+            return False
         return True
 
     return False