home-environment: reset PATH in activation script

Starting with state version 22.11 we completely reset the PATH
variable in the activation script. This is to avoid impurities and
unexpected results if the activation script accidentally uses a
command found in the user's PATH.

nixos/common.nix

@@ -35,7 +35,7 @@ let
 
           # Make activation script use same version of Nix as system as a whole.
           # This avoids problems with Nix not being in PATH.
-          home.extraActivationPath = [ config.nix.package ];
+          nix.package = config.nix.package;
         };
       })
     ] ++ cfg.sharedModules;