Fixes <http://bugs.gnu.org/25242>.
Reported by Leo Famulari <leo@famulari.name>.
The regression was introduced in 94d92c7796.
* nix/libstore/build.cc (DerivationGoal::startBuilder): Set 'useChroot'
as a function 'of isBuiltin(drv)'.
(DerivationGoal::runChild): Use 'useChroot' instead of
'useChroot && !isBuiltin(drv)'.