Put back nicer copyClosure instead of manual closure + copy

It looks like we accidentally got the old code back, probably after a merge
conflict resolution.

src/hydra-queue-runner/queue-monitor.cc

@@ -535,9 +535,9 @@ Step::ptr State::createStep(ref<Store> destStore,
                             // FIXME: should copy directly from substituter to destStore.
                         }
 
-                        StorePathSet closure;
-                        localStore->computeFSClosure({*path}, closure);
-                        copyPaths(*localStore, *destStore, closure, NoRepair, CheckSigs, NoSubstitute);
+                        copyClosure(*localStore, *destStore,
+                            StorePathSet { *path },
+                            NoRepair, CheckSigs, NoSubstitute);
 
                         time_t stopTime = time(0);