add NUXT_ namespace to the env name

Background

While getting intouch with mcp and ai sdk framework, I'm stumble about the not optimal used env naming of the nuxt getting started part. Since the env is only available at build but not at run time, when not NUXT_ as prefixed is used.

So I added the NUXT_* prefix to the doc, evade other followers to stumble about the point.

Official doc about this topic: https://nuxt.com/docs/guide/going-further/runtime-config#environment-variables

Keeping the note aboud the default env name unchanged. In https://github.com/vercel/ai/blob/main/examples/nuxt-openai/.env.example the right name already used.

Summary

Change env name of nuxt getting started section to NUXT_*