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/get-drvs.cc

@@ -171,7 +171,9 @@ StringSet DrvInfo::queryMetaNames() {
   if (!getMeta()) {
     return res;
   }
-  for (auto& i : *meta) res.insert(i.name);
+  for (auto& i : *meta) {
+    res.insert(i.name);
+  }
   return res;
 }