nix-ld: disable build on non-linux platforms

pkgs/os-specific/linux/nix-ld/default.nix

···
     homepage = "https://github.com/Mic92/nix-ld";
     license = licenses.mit;
     maintainers = with maintainers; [ mic92 ];
-    platforms = platforms.unix;
+    platforms = platforms.linux;
   };
 }