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/primops.cc

@@ -561,7 +561,7 @@ static void prim_derivationStrict(EvalState& state, const Pos& pos,
   StringSet outputs;
   outputs.insert("out");
 
-  for (auto& i : args[0]->attrs->lexicographicOrder()) {
+  for (auto& i : args[0]->attrs->SortedByKeys()) {
     if (i->name == state.sIgnoreNulls) {
       continue;
     }