Enable Steam portal integration

This change configures the system to use the GTK portal for desktop integration with Steam.

It ensures that the necessary portal file is available and sets an environment variable to enable Steam's portal functionality. This should improve how Steam interacts with the desktop environment for features like file pickers and screen sharing.

nixos/gaming/steam/default.nix

@@ -34,7 +34,11 @@
         pkgsi686Linux.mesa
       ];
     };
+    environment.sessionVariables = {
+      STEAM_USE_PORTAL = "1";
+    };
     gamescopeSession.enable = true;
   };
   programs.bash.enable = true;
+
 }