[ci] Only run sizebot on pull requests
Oops, forgot to port this bit over. ghstack-source-id: 7fae8021d80a2ea58e89759147553e6910b49a0f Pull Request resolved: https://github.com/facebook/react/pull/30390
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Lauren Tan committed
3f4391d5896b419b93f58103d153b67906460c2a
Parent: fd2b3e1