E.g. http://hydra/build/3515983/output/out downloads the output named "out" as a bzip2-compressed NAR.
45A23NZPOQOJD4R45FPVPJR3642OCVIJHBA4RBQ3IB7JAQ2FGI4QC
error($c, "This build is not finished yet.") unless $build->finished;
my $output = $build->buildoutputs->find({name => $outputName});
notFound($c, "This build has no output named ‘$outputName’") unless defined $output;
error($c, "Output is not available.") unless isValidPath $output->path;
$c->response->header('Content-Disposition', "attachment; filename=\"build-${\$build->id}-${\$outputName}.nar.bz2\"");
$c->stash->{current_view} = 'NixNAR';
$c->stash->{storePath} = $output->path;
}