gcc warning fix

ostd/process.hh

@@ -404,6 +404,8 @@ private:
     void open_full(
         string_range cmd, InputRange1 args, InputRange2 env, bool use_path
     ) {
+        /* avoid false-positive warning with GCC */
+        static_cast<void>(env);
         static_assert(
             std::is_constructible_v<
                 string_range, range_reference_t<InputRange1>