We're migrating our Next.js app from Pages to Workers through OpenNext, and we're trying to set up preview URLs / non-production brunch deploys. In Pages, there's an option to set different variables for production and non-production deploys, but I don't see such an option in Workers. As far as I understand, we can have two environments in Wrangler, but that only allows us to set per-env runtime variables, not build variables. Has anyone figured out how to set it up?