dillon1000/react

[compiler] Check if current branch is main

This script needs to run from `main` since it commits version bumps for
packages, and those need to point to publicly available hashes. So,
throw an error if we're not already on main.

ghstack-source-id: ce0168e826b990fd55733e777ee8effe4f35400a
Pull Request resolved: https://github.com/facebook/react/pull/29083