message: Fix non-deterministic --cfg feature flag list generation
  The resolved features were returned in a HashSet and then iterated over,
  which returns the feature names in arbitrary order.
  See
diff: @@ -601,7 +601,7 @@ fn rustdoc(cx: &mut Context, unit: &Unit) -> CargoResult<Work> {
  rustdoc.arg("-o").arg(doc_dir);
  - for feat in cx.resolve.features(unit.pkg.package_id()) {
  + for feat in cx.resolve.features_sorted(unit.pkg.package_id()) {
  rustdoc.arg("--cfg").arg(&format!("feature=\"{}\"", feat));
  }
  @@ -770,7 +770,7 @@ fn build_base_args(cx: &mut Context,
  cmd.arg("--cfg").arg("test");
  }
  - for feat in cx.resolve.features(unit.pkg.package_id()).iter() {
  + for feat in cx.resolve.features_sorted(unit.pkg.package_id()) {
  cmd.arg("--cfg").arg(&format!("feature=\"{}\"", feat));
  }
  