How to change build result (~/derivative-maker) to another custom build folder?

I am trying to build with a VM and the disk space is low. I moved to an external drive shared with it, and it is still trying to put everything under /home. Where can i set the output directory?

Documented just now:
Build Variables

Thanks. I was able to just set HOMEVAR before executing derivitake-maker. That seems like a very easy way to do it.