Fixes <http://bugs.gnu.org/25242>. Reported by Leo Famulari <leo@famulari.name>. The regression was introduced in 94d92c7796a3dd50c27d532315f7d497ac99f08e. * nix/libstore/build.cc (DerivationGoal::startBuilder): Set 'useChroot' as a function 'of isBuiltin(drv)'. (DerivationGoal::runChild): Use 'useChroot' instead of 'useChroot && !isBuiltin(drv)'.