[ci] Try sharding test_build

The tests for the build currently take about 3.5 minutes and is at the
moment the long pole for the whole workflow. By sharding we can make
this workflow faster and save about a minute or so in overall wall time.

ghstack-source-id: c23df7b6d7fd2df857abb976546456ca874d9151
Pull Request resolved: https://github.com/facebook/react/pull/30349