Don't override SteamGameId if it's already set

Valve's proton wine has code that checks for SteamGameId for specific games.
This is required for alternate code paths for those games. Some examples include Red Dead Redemption 2 and GTA V.
For the non-steam versions of those games we have to define SteamGameId as an envvar with the actual Steam Id value
in order for those code changes to take effect, therefore we cannot assign SteamGameId to 'lutris-game' for all games.

lutris/runners/wine.py

@@ -937,7 +937,8 @@ def get_env(self, os_env=False, disable_runtime=False):
             env["STEAM_COMPAT_DATA_PATH"] = self.prefix_path
             env["STEAM_COMPAT_APP_ID"] = '0'
             env["SteamAppId"] = '0'
-            env["SteamGameId"] = "lutris-game"
+            if not ("SteamGameId" in env):
+                env["SteamGameId"] = "lutris-game"
         return env
 
     def get_runtime_env(self):