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/names.cc

@@ -99,7 +99,9 @@ int compareVersions(const string& v1, const string& v2) {
 
 DrvNames drvNamesFromArgs(const Strings& opArgs) {
   DrvNames result;
-  for (auto& i : opArgs) result.push_back(DrvName(i));
+  for (auto& i : opArgs) {
+    result.push_back(DrvName(i));
+  }
   return result;
 }