ci: add build script to frontend package.json

[adgad]

Thanks for submitting a PR! Please check the boxes below:

• [x] I have run pre-commit to check linting
• [ ] I have added information to docs/ if required so people know about the feature!
• [x] I have filled in the "Changes" section below?
• [x] I have filled in the "How did you test this code" section below?
• [x] I have used a Conventional Commit title for this Pull Request

Changes

Currently when deploying Flagsmith to Heroku, the frontend fails to start because the build step hasn't run.

Heroku will automatically run a build script from the package.json, so adding this alias will allow the app to be deployed to Heroku with the node.js buildpack.

https://devcenter.heroku.com/articles/nodejs-support#customizing-the-build-process

How did you test this code?

Deployed a fork with this line to Heroku, using the heroku/nodejs buildpack, and confirmed the build step ran and the app worked as expected.

[vercel[bot]]

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1 Ignored Deployment

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/docs/GcMtGrQL7ZP7AoE13nBGzn8jtetf)) | [Visit Preview](https://docs-git-fork-financial-times-add-heroku-build-step-flagsmith.vercel.app) | | Jun 25, 2024 8:34am |

[vercel[bot]]

@adgad is attempting to deploy a commit to the Flagsmith Team on Vercel.

A member of the Team first needs to authorize it.

[matthewelwell]

Thanks a lot for this @adgad ! I've updated the title to conform to our conventional commit restrictions (you can see them here). I'll re-run the workflows and get this merged as soon as we can.

[github-actions[bot]]

Uffizzi Preview deployment-53442 was deleted.