nix-darwin: Find Nix with restrictive sudoers PATH.

If the system has a restrictive default PATH in sudoers, Nix might not
be available within the activation script. Since our script sets its
own PATH (except for inherting Nix), this should only impact looking
up Nix itself.

Author: shlevy

nix-darwin/default.nix

@@ -19,7 +19,7 @@ in
       system.activationScripts.postActivation.text = lib.concatStringsSep "\n" (
         lib.mapAttrsToList (username: usercfg: ''
           echo Activating home-manager configuration for ${usercfg.home.username}
-          launchctl asuser "$(id -u ${usercfg.home.username})" sudo -u ${usercfg.home.username} --set-home ${pkgs.writeShellScript "activation-${usercfg.home.username}" ''
+          launchctl asuser "$(id -u ${usercfg.home.username})" sudo -u ${usercfg.home.username} --preserve-env=PATH --set-home ${pkgs.writeShellScript "activation-${usercfg.home.username}" ''
             ${lib.optionalString (
               cfg.backupFileExtension != null
             ) "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}