fix(v2): always exit after successful build

[mohrash92]

Motivation

When running yarn build, a successful build does not always exit as highlighted in the following issue #2431. We can achieve this by exiting when the build has finished.

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

1. Cloned repo and reproduced exiting problem as shown in [v2] yarn build hangs forever after build is completed successfully #2431
2. Went into for v2 website for docusaurus and ran yarn and yarn build
3. Replaced contents of build.js of hookstate with the one from docusaurus v2
4. Ran yard build and the process exits instead of not exiting after build is successful
5. Also ran yarn build --out-dir build/nested to ensure that the nested folder is generated and it works.

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[docusaurus-bot]

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[lex111]

This comes from async operations (e.g. setInterval in custom React component). But in any case, the building must finishes successfully therefore LGTM.

[yangshun]

Many thanks!