Use fmt::localtime instead of thread-unsafe std::localtime

fmt::localtime is also less awkward to use compared to std::localtime.

Source/Core/Core/Core.cpp

@@ -739,7 +739,7 @@ static std::string GenerateScreenshotName()
 
   const std::time_t cur_time = std::time(nullptr);
   const std::string base_name =
-      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, *std::localtime(&cur_time));
+      fmt::format("{}_{:%Y-%m-%d_%H-%M-%S}", path_prefix, fmt::localtime(cur_time));
 
   // First try a filename without any suffixes, if already exists then append increasing numbers
   std::string name = fmt::format("{}.png", base_name);