fix(3p/nix/libexpr): Fix post-increment operator in BindingsIterator

We accidentally returned the incremented iterator in the
post-increment, this fixes it.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I36c79eb56359bb12a78ad3489e7d7d2eb2053510
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1140
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/attr-set.cc

@@ -23,12 +23,13 @@ BindingsIterator& BindingsIterator::operator++() {
 }
 
 BindingsIterator BindingsIterator::operator++(int) {
+  auto old = *this;
   std::visit(util::overloaded{
                  [](AttributeMap::iterator& iter) { iter++; },
                  [](AttributeVector::iterator& iter) { iter++; },
              },
              _iterator);
-  return *this;
+  return old;
 }
 
 bool BindingsIterator::operator==(const BindingsIterator& other) const {