Fix "Not an ARRAY reference" error

It's a mystery why this error is not triggered in the build farm
(e.g. http://hydra.nixos.org/build/3265602).  Ah well.

scripts/download-from-binary-cache.pl.in

@@ -273,7 +273,7 @@ sub processNARInfo {
     $insertNAR->execute(
         $cache->{id}, basename($storePath), $narInfo->{url}, $narInfo->{compression},
         $narInfo->{fileHash}, $narInfo->{fileSize}, $narInfo->{narHash}, $narInfo->{narSize},
-        join(" ", @$narInfo->{refs}), $narInfo->{deriver}, $narInfo->{system}, time())
+        join(" ", @{$narInfo->{refs}}), $narInfo->{deriver}, $narInfo->{system}, time())
         unless $request->{url} =~ /^file:/;
 
     return $narInfo;