go-ios: fix build on darwin

Since e03feee6e00e6867b243a1138565a4d841e1c2cf, the package applies
patches to replace paths for Linux utilities. However, these are not
available for Darwin.

pkgs/by-name/go/go-ios/package.nix

···
 {
   lib,
+  stdenv,
   buildGoModule,
   fetchFromGitHub,
   nix-update-script,
···
     "restapi"
   ];
 
-  postPatch = ''
+  postPatch = lib.optionalString stdenv.hostPlatform.isLinux ''
     substituteInPlace ncm/linux_commands.go \
       --replace-fail "ip " "${lib.getExe' iproute2 "ip"} "