Prevent multiple builds with the same (job, outPath) tuple from being added

This happened in a pathological case in Nixpkgs: the "grub" job is
evaluated for i686-linux and x86_64-linux, but in the latter case it
returns the same derivation as in the former case.  So only one build
should be added.

src/lib/Hydra/Controller/Build.pm

@@ -532,7 +532,7 @@ sub clone_submit : Chained('build') PathPart('clone/submit') Args(0) {
     my %currentBuilds;
     my $newBuild = checkBuild(
         $c->model('DB'), $build->project, $build->jobset,
-        $inputInfo, $nixExprInput, $job, \%currentBuilds, undef);
+        $inputInfo, $nixExprInput, $job, \%currentBuilds, undef, {});
 
     error($c, "This build has already been performed.") unless $newBuild;