Although a completely empty DOOMDIR seems to work (at least for build
purposes), I doubt Doom actually supports this. And it requires some
extra checks on our side as well.
Just check in a trivial DOOMDIR and use that instead.
Exposing it as a package was (at least according to `nix flake check`)
incorrect.
This means we now use the user's nixpkgs's `pkgs.callPackage` instead of
our own, but I think that's ok.
Although not used much with doomProfile set, the default (.local in the
Doom source tree) will not work.
Currently straight-base-dir ends up relative to this, although that may
change.
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.
