Fix issues with the Steam flatpak runner

lmm/runners/steam.py

@@ -1,8 +1,9 @@
 import abc
 from pathlib import Path
 
+from lmm.cmd import run_cmd_nonblocking
 from lmm.runners.runner import Runner
-from lmm.games.game import Game
+from lmm.games.game import Game, ProtonGame
 
 
 class SteamRuntime(abc.ABC):