Make --repair work on Darwin

Mac OS X doesn't allow renaming a read-only directory.

http://hydra.nixos.org/build/9113895

src/libstore/build.cc

@@ -2810,12 +2810,12 @@ void SubstitutionGoal::finished()
         return;
     }
 
-    canonicalisePathMetaData(destPath, -1);
-
-    worker.store.optimisePath(destPath); // FIXME: combine with hashPath()
-
     if (repair) replaceValidPath(storePath, destPath);
 
+    canonicalisePathMetaData(storePath, -1);
+
+    worker.store.optimisePath(storePath); // FIXME: combine with hashPath()
+
     ValidPathInfo info2;
     info2.path = storePath;
     info2.hash = hash.first;