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/attr-set.hh

@@ -55,8 +55,12 @@ class Bindings {
   iterator begin();
   iterator end();
 
-  // TODO: can callers just iterate?
-  [[deprecated]] std::vector<const Attr*> lexicographicOrder();
+  // Returns the elements of the attribute set as a vector, sorted
+  // lexicographically by keys.
+  //
+  // This is used primarily for builtins that have guaranteed
+  // ordering, such as `attrNames` or `attrValues`.
+  std::vector<const Attr*> SortedByKeys();
 
   // oh no
   friend class EvalState;