fix(nuxt): support usage of std-env in runtime code

[danielroe]

🔗 Linked issue

resolves #21313

❓ 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

We should transpile std-env since if it is used in runtime code the process.env variables need to be statically replaced with unenv mock or hardcoded values.

We can also statically replace process.env.NODE_ENV within client code for this use case.

📝 Checklist

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