steam: add xrandr binary to env

Games utilizing LWJGL >= 2.4 && < 3.0 need this as the framework parses
display information from the command line output of xrandr[1] on Linux.
There are a number of LWJGL games on Steam currently.

[1]: 46f602f0c6/src/java/org/lwjgl/opengl/XRandR.java (L72)

pkgs/games/steam/chrootenv.nix

@@ -13,6 +13,7 @@ buildFHSUserEnv {
       pkgs.python
       pkgs.gnome2.zenity
       pkgs.xdg_utils
+      pkgs.xlibs.xrandr
     ]
     ++ lib.optional (config.steam.java or false) pkgs.jdk
     ++ lib.optional (config.steam.primus or false) pkgs.primus