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/libexpr/attr-set.hh

@@ -70,7 +70,9 @@ class Bindings {
   std::vector<const Attr*> lexicographicOrder() const {
     std::vector<const Attr*> res;
     res.reserve(size_);
-    for (size_t n = 0; n < size_; n++) res.emplace_back(&attrs[n]);
+    for (size_t n = 0; n < size_; n++) {
+      res.emplace_back(&attrs[n]);
+    }
     std::sort(res.begin(), res.end(), [](const Attr* a, const Attr* b) {
       return (const string&)a->name < (const string&)b->name;
     });