daemon: Allow check builds of 'builtin:download' derivations.

daemon: Allow check builds of 'builtin:download' derivations.

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.