linux-libre: mark as broken

- broken since 2020-12-25
- slows down nixpkgs-review
- maintainer has been pinged but did not reacted: https://github.com/NixOS/nixpkgs/issues/111594

pkgs/os-specific/linux/kernel/linux-libre.nix

@@ -35,6 +35,8 @@ in linux.override {
       '';
     };
 
+    extraMeta.broken = true;
+
     passthru.updateScript = ./update-libre.sh;
 
     maintainers = [ lib.maintainers.qyliss ];