steam: fix for new fhs-userenv

pkgs/games/steam/chrootenv.nix

···
     export LD_PRELOAD=/lib32/libpulse.so:/lib64/libpulse.so:/lib32/libasound.so:/lib64/libasound.so
   '';
 
-  runScript = "exec steam";
+  runScript = "steam";
 }