This flag allows the user to optionally exclude
switch-to-confguration.pl from toplevel.
This is interesting for appliance images where you don't want to re-build
the system. This flag is called `rebuildable` because the standard
interface to do this is `nixos-rebuild` which will not work anymore with
this change.
This helps with understanding the code.
We might make this not depend on environment variables later.
systemBuilderArgs is a form of global state, which isn't helpful.
