style(3p/nix): Add braces around single-line for-loops

These were not caught by the previous clang-tidy invocation, but were
instead sorted out using amber[0] as such:

    ambr --regex 'for (\(.+\))\s([a-z].*;)' 'for $1 { $2 }'

[0]: https://github.com/dalance/amber

third_party/nix/src/libutil/thread-pool.hh

@@ -125,7 +125,9 @@ void processGraph(ThreadPool& pool, const std::set<T>& nodes,
     }
   };
 
-  for (auto& node : nodes) pool.enqueue(std::bind(worker, std::ref(node)));
+  for (auto& node : nodes) {
+    pool.enqueue(std::bind(worker, std::ref(node)));
+  }
 
   pool.process();