Fix missing trailing / in commit artifacts workflow

The trailing / was being omitted, so instead of moving the cjs
directory itself, it would move only its contents instead. This broke
some internal path assumptions.

Additionally, updates the step to create the react-dom directory prior
to moving.

ghstack-source-id: b6eedb0c88cd3aa3a786a3d3d280ede5ee81a063
Pull Request resolved: https://github.com/facebook/react/pull/31026