fix: use unescaped %I in ntfy failure notification script

%i passes the escaped unit name which systemctl status cannot resolve,
causing "Failed to mangle name" errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/nixos/services/systemd-email-notify/default.nix

@@ -117,7 +117,7 @@ in
               ${cfg.ntfy.url}/${cfg.ntfy.topic}
           '';
         };
-        scriptArgs = "%i";
+        scriptArgs = "%I";
       };
     })
   ]);