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/value-to-json.cc

@@ -50,7 +50,9 @@ void printValueAsJSON(EvalState& state, bool strict, Value& v,
       if (i == v.attrs->end()) {
         auto obj(out.object());
         StringSet names;
-        for (auto& j : *v.attrs) names.insert(j.name);
+        for (auto& j : *v.attrs) {
+          names.insert(j.name);
+        }
         for (auto& j : names) {
           Attr& a(*v.attrs->find(state.symbols.create(j)));
           auto placeholder(obj.placeholder(j));