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/primops/context.cc

@@ -35,7 +35,9 @@ static void prim_unsafeDiscardOutputDependency(EvalState& state, const Pos& pos,
   string s = state.coerceToString(pos, *args[0], context);
 
   PathSet context2;
-  for (auto& p : context) context2.insert(p.at(0) == '=' ? string(p, 1) : p);
+  for (auto& p : context) {
+    context2.insert(p.at(0) == '=' ? string(p, 1) : p);
+  }
 
   mkString(v, s, context2);
 }