linux_6_1: rebuild on x86_64-linux

Just one-off work around these hash mismatches:
https://hydra.nixos.org/build/216994032#tabs-buildsteps
(I don't know a better way, I'm afraid)

pkgs/os-specific/linux/kernel/linux-6.1.nix

···
     url = "mirror://kernel/linux/kernel/v6.x/linux-${version}.tar.xz";
     sha256 = "149h95r5msvqah868zd36y92ls9h41cr1rb5vzinl20mxdn46wnb";
   };
+  # TODO: possible to remove after any rebuild, e.g. after update.
+  extraConfig = lib.optionalString (buildPackages.system == "x86_64-linux") "\n";
 } // (args.argsOverride or { }))