dillon1000/react

Use --frozen-lockfile for Yarn in CI build (#12914)

CI builds should always use the `--frozen-lockfile` option. It will fail the build if the lockfile is out-of-date:

> If you need reproducible dependencies, which is usually the case with the continuous integration systems, you should pass --frozen-lockfile flag.

(https://yarnpkg.com/en/docs/cli/install/)