games/steam-sandbox: Add mangohud

users/simon/modules/games.nix

@@ -60,6 +60,7 @@ let
         --bind "$base_dir/.config" "$HOME/.config"
         --bind "$base_dir/.factorio" "$HOME/.factorio"
         --bind "$base_dir/data" "$HOME/data"
+        --ro-bind-try "$HOME/.config/MangoHud" "$HOME/.config/MangoHud"
 
         # input
         --dev-bind /dev/input /dev/input
@@ -68,6 +69,7 @@ let
       )
 
     unset SDL_VIDEODRIVER QT_QPA_PLATFORM # games generally don’t support wayland
+    export PATH="${pkgs.unstable.mangohud}/bin:$PATH"
 
     ${pkgs.bubblewrap}/bin/bwrap \
         "''${bubblewrap_args[@]}" \