fix: support nitro preset detection in zero config environments

[HADB]

🔗 Linked issue

resolves #213

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

After some digging, I found out that nuxt.options.nitro.preset was undefined in my cloudflare build environment with zero-config support of nitro, but there's an environment variable NITRO_PRESET:"cloudflare_pages" we can use to detect the nitro preset just as nitro doing itself: unjs/nitro: src/core/config/loader.ts

[antfu]

Thanks you!