ci: verify JS build artifacts aren't committed (#48091)

Summary:
Building our JS packages dirties the repo. This makes sure we don't
accidentally commit these to the repo, as it'll break Flow tests with an
obscure error message.

Changelog: [Internal]

Pull Request resolved: https://github.com/facebook/react-native/pull/48091

Test Plan:
Ran this locally, the GH lint action should run on this PR.
I'll intentionally add a build artifact to validate.
| A | B | C |
| - | - | - |
| ![CleanShot 2024-12-04 at 14 11 16@2x](https://github.com/user-attachments/assets/11c05c32-12c8-4d85-9a28-b3bdffb42ea0) | ![CleanShot 2024-12-04 at 14 11 22@2x](https://github.com/user-attachments/assets/b4c4f1dc-2cb9-4138-8931-13e71c015a6d) | ![CleanShot 2024-12-04 at 14 19 31@2x](https://github.com/user-attachments/assets/953bf783-57ba-4832-bbd8-b36e23a2ccb4) |

Reviewed By: cipolleschi

Differential Revision: D66760144

Pulled By: blakef

fbshipit-source-id: 81f20fa0a83d5f17b5773d1608664b683ba74409