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/get-drvs.cc

@@ -387,7 +387,7 @@ static void getDerivations(EvalState& state, Value& vIn,
        there are names clashes between derivations, the derivation
        bound to the attribute with the "lower" name should take
        precedence). */
-    for (auto& i : v.attrs->lexicographicOrder()) {
+    for (auto& i : v.attrs->SortedByKeys()) {
       DLOG(INFO) << "evaluating attribute '" << i->name << "'";
       if (!std::regex_match(std::string(i->name), attrRegex)) {
         continue;