nix-info: use `stdenv.hostPlatform.system`

`pkgs.system` is an alias since 4246d6ce21d2d8d33e2d30f42b3d9d446c5dc143.

pkgs/tools/nix/info/info.sh

···
 }
 
 desc_system() {
-    nixev '(import <nixpkgs> {}).system'
+    nixev '(import <nixpkgs> {}).stdenv.hostPlatform.system'
 }
 
 desc_host_os() {