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]: https://github.com/LWJGL/lwjgl/blob/46f602f0c680a92349b4185f886609200d614990/src/java/org/lwjgl/opengl/XRandR.java#L72