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-xml.cc

@@ -29,7 +29,9 @@ static void showAttrs(EvalState& state, bool strict, bool location,
                       PathSet& drvsSeen) {
   StringSet names;
 
-  for (auto& i : attrs) names.insert(i.name);
+  for (auto& i : attrs) {
+    names.insert(i.name);
+  }
 
   for (auto& i : names) {
     Attr& a(*attrs.find(state.symbols.create(i)));