nixos/steam: fix eval after #293564

      apply = steam: steam.override (prev: {

        extraEnv = (lib.optionalAttrs (cfg.extraCompatPackages != [ ]) {

            STEAM_EXTRA_COMPAT_TOOLS_PATHS = makeBinPath cfg.extraCompatPackages;

          }) // prev.extraEnv;

          }) // (prev.extraEnv or {});

        extraLibraries = pkgs: let

          prevLibs = if prev ? extraLibraries then prev.extraLibraries pkgs else [ ];

          additionalLibs = with config.hardware.opengl;