* Beginning of secure multi-user Nix stores. If Nix is started as

  root (or setuid root), then builds will be performed under one of
  the users listed in the `build-users' configuration variables.  This
  is to make it impossible to influence build results externally,
  allowing locally built derivations to be shared safely between
  users (see ASE-2005 paper).

  To do: only one builder should be active per build user.