Having this empty only really makes sense for minimal/full test builds.
Also, /var/empty was not doing what it looks like (it's not accessible
from inside the build sandbox). Use nixpkgs's emptyDirectory instead.
I was hoping to avoid this but it does not seem practical: I'm pretty
sure I need the user module in the store to override its packages.el,
and Doom does not separate the user module and doom-user-dir.
