before (BOOKING_URL, no prefix), client bundle around the click handler:
  onClick:()=>o(e.default.env.BOOKING_URL+"/book")
  -> a lookup on the browser's process.env at click time; nothing was filled in at build time
after (NEXT_PUBLIC_BOOKING_URL), client bundle around the click handler:
  onClick:()=>o("https://booking.example.test/book")
  -> the value was written into the bundle by `next build`
(from: grep -rho '.\{60\}/book.\{10\}' before/.next/static after/.next/static)