Use xctool log dir if provided in environment
Use xctool log dir if provided in environment

Summary: `buck` will provide this environment variable when launching `fbxctest`. We can use it here as a log directory.

Reviewed By: marekcirkos

Differential Revision: D4572155

fbshipit-source-id: 3dcccb4f7e27eb40d8eb3ceef644bb35b3aba00f

StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl