key_manager: Make use of IOFile in WriteKeyToFile()

This properly handles unicode-based paths on Windows, while opening a
raw stream doesn't out-of-the-box.

Prevents file creation from potentially failing on Windows PCs that make
use of unicode characters in their save paths (e.g. writing to a user's
AppData folder, where the user has a name with non-ASCII characters).

src/core/crypto/key_manager.cpp

@@ -668,23 +668,27 @@ void KeyManager::WriteKeyToFile(KeyCategory category, std::string_view keyname,
                                 const std::array<u8, Size>& key) {
     const std::string yuzu_keys_dir = FileUtil::GetUserPath(FileUtil::UserPath::KeysDir);
     std::string filename = "title.keys_autogenerated";
-    if (category == KeyCategory::Standard)
+    if (category == KeyCategory::Standard) {
         filename = dev_mode ? "dev.keys_autogenerated" : "prod.keys_autogenerated";
-    else if (category == KeyCategory::Console)
+    } else if (category == KeyCategory::Console) {
         filename = "console.keys_autogenerated";
-    const auto add_info_text = !FileUtil::Exists(yuzu_keys_dir + DIR_SEP + filename);
-    FileUtil::CreateFullPath(yuzu_keys_dir + DIR_SEP + filename);
-    std::ofstream file(yuzu_keys_dir + DIR_SEP + filename, std::ios::app);
-    if (!file.is_open())
+    }
+
+    const auto path = yuzu_keys_dir + DIR_SEP + filename;
+    const auto add_info_text = !FileUtil::Exists(path);
+    FileUtil::CreateFullPath(path);
+    FileUtil::IOFile file{path, "a"};
+    if (!file.IsOpen()) {
         return;
+    }
     if (add_info_text) {
-        file
-            << "# This file is autogenerated by Yuzu\n"
-            << "# It serves to store keys that were automatically generated from the normal keys\n"
-            << "# If you are experiencing issues involving keys, it may help to delete this file\n";
+        file.WriteString(
+            "# This file is autogenerated by Yuzu\n"
+            "# It serves to store keys that were automatically generated from the normal keys\n"
+            "# If you are experiencing issues involving keys, it may help to delete this file\n");
     }
 
-    file << fmt::format("\n{} = {}", keyname, Common::HexToString(key));
+    file.WriteString(fmt::format("\n{} = {}", keyname, Common::HexToString(key)));
     AttemptLoadKeyFile(yuzu_keys_dir, yuzu_keys_dir, filename, category == KeyCategory::Title);
 }