nixos: require pkgs.which

This properly implements revert in
0729f606973870c03d21bb2f21b70d91216943ca.

We used to have which='type -P' alias, but really it's best to just
rely on which package, only 88K in size.

cc @edolstra

nixos/modules/config/system-path.nix

@@ -40,6 +40,7 @@ let
       pkgs.time
       pkgs.texinfoInteractive
       pkgs.utillinux
+      pkgs.which # 88K size
     ];
 
 in