Require build to succeed before preview deployment

[taesungh]

Stemming from #205 and related to #206, to further reduce the number of deployments on Vercel, we can modify our Actions workflow to run the preview deployment for pull requests only if next build runs successfully. This will mean it takes a little longer for a successful preview deployment to be available but will help reduce the amount of unnecessary failed deployments which go into our daily quota of 100. This should also help tech team developers be more aware of what went wrong during a deployment since the build logs in Vercel account aren't accessible to everyone, but the Actions workflow logs should be.

Regarding the build time, we might be able to take advantage of Vercel's remote cache as they describe a way to Use Remote Caching from external CI/CD. If this works out, I think the build artifacts from the Actions workflow can be restored from cache during the preview deployment, hopefully making the time increase negligible.