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/libstore/builtins/buildenv.cc

@@ -210,7 +210,9 @@ void builtinBuildenv(const BasicDerivation& drv) {
   while (!postponed.empty()) {
     auto pkgDirs = postponed;
     postponed = FileProp{};
-    for (const auto& pkgDir : pkgDirs) addPkg(pkgDir, priorityCounter++);
+    for (const auto& pkgDir : pkgDirs) {
+      addPkg(pkgDir, priorityCounter++);
+    }
   }
 
   LOG(INFO) << "created " << symlinks << " symlinks in user environment";