refactor(3p/nix/libexpr): Use absl::btree_map::merge for '//'

Instead of doing some sort of inline merge-sort of the two attribute
sets, use the attribute sets merge function.

This commit alone does not build and is not supposed to.

third_party/nix/src/libexpr/attr-set.hh

@@ -50,6 +50,9 @@ class Bindings {
   iterator begin();
   iterator end();
 
+  // Merge values from other into the current attribute
+  void merge(Bindings* other);
+
   // ???
   [[deprecated]] void sort();