[ci] Fix prereleases reusable workflow

Turns out I had configured the reusable workflow with the wrong `on`
command.

Also removes the workflow_dispatch config from the nightly workflow as
that was not meant to be triggered manually.

ghstack-source-id: 426d07279d1a2b106918fbf662d4c64e169f1ef7
Pull Request resolved: https://github.com/facebook/react/pull/30498