Fixes <http://bugs.gnu.org/25089>. Reported by Leo Famulari <leo@famulari.name>. * nix/libstore/build.cc (DerivationGoal::runChild): In the 'isBuiltin' case, check whether DRV's output is in 'redirectedOutputs', and pass an 'output' argument to the built-in builder. (DerivationGoal::addHashRewrite): Add 'printMsg' call. * nix/libstore/builtins.hh (derivationBuilder): Add 'output' parameter. * nix/libstore/builtins.cc (builtinDownload): Likewise. Add OUTPUT to ARGV. * guix/scripts/perform-download.scm (perform-download): Add 'output' parameter. (guix-perform-download): Adjust 'match' clauses accordingly. * tests/derivations.scm ("'download' built-in builder, check mode"): New test.