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/symbol-table.hh

@@ -54,7 +54,9 @@ class SymbolTable {
 
   template <typename T>
   void dump(T callback) {
-    for (auto& s : symbols) callback(s);
+    for (auto& s : symbols) {
+      callback(s);
+    }
   }
 };