refactor(3p/nix): Rename & undeprecate Bindings::lexicographicOrder

The function is renamed to `SortedByKeys`, which is more descriptive,
and annotated with a comment about what it is used for.

The deprecation warning has been removed because this function is
currently functionally required.

Change-Id: I0ee3a76deff05f366feca9ddac8f38ab34bffbd0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1288
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/eval.cc

@@ -89,7 +89,7 @@ static void printValue(std::ostream& str, std::set<const Value*>& active,
       break;
     case tAttrs: {
       str << "{ ";
-      for (auto& i : v.attrs->lexicographicOrder()) {
+      for (auto& i : v.attrs->SortedByKeys()) {
         str << i->name << " = ";
         printValue(str, active, *i->value);
         str << "; ";