Disable remaining implicit conversions

src/os.cc

@@ -167,10 +167,10 @@ void detail::format_windows_error(detail::buffer<char>& out, int error_code,
   FMT_TRY {
     system_message msg(error_code);
     if (msg) {
-      utf16_to_utf8 utf8_message;
+      auto utf8_message = utf16_to_utf8();
       if (utf8_message.convert(msg) == ERROR_SUCCESS) {
         fmt::format_to(buffer_appender<char>(out), "{}: {}", message,
-                       utf8_message);
+                       string_view(utf8_message));
         return;
       }
     }