Hopefully someone a bit more experienced with xlights can comment but I can't find a simple "box" to change the caching directory I think its solidly defined in the code.
So far from a real quick look it seems that the defined location for cache files are the show directory. Which you may be able to manually define in RenderCache.cpp and recompile xlights but im not sure what else relates to this directory.
You could just disable the function. Doing this will just add a bit more time to your rendering.