vendor: update buildkit to master@2d91ddcceedc

Signed-off-by: Justin Chadwell <me@jedevc.com>
(cherry picked from commit 0a7f96cbfb)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

tests/workers/docker-container.go

@@ -64,3 +64,7 @@ func (w *containerWorker) New(ctx context.Context, cfg *integration.BackendConfi
 		builder: name,
 	}, cl, nil
 }
+
+func (w *containerWorker) Close() error {
+	return nil
+}