Rebuild labels are wrong for limited platforms

[Stanisław Pitucha]

In some cases, usually when the platform is explicitly set, ofborg labels for rebuilds are wrong. For example ethtool is limited to linux only (https://github.com/NixOS/nixpkgs/blob/94705be41876f2d5ae73f562ad93c54ba6d205f2/pkgs/tools/misc/ethtool/default.nix#L46), but the update PR shows the multiple darwin rebuilds: NixOS/nixpkgs#195398