feat: add NITRO_TIMING env support

[atinux]

❓ 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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I found it useful to debug in production by simply setting NITRO_TIMING=1 env variable 🚀

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

We can properly support config override with env for all keys with (unjs/c12#16). In the meantime, you can use timing: Boolean(process.env.NITRO_CONFIG_TIMING) inside nitro config.