fix(vite-node): wrong entry been resolved in stub mode

[antfu]

🔗 Linked issue

Fixes #3974 (comment)

process.server was undefined so it resolved to the client entry and caused loaded app is an function instead of a server-rendered app.

This only happens on the stub mode of development, but not the production build.
TBH I am not sure why it's happening and if this is the correct fix.

❓ 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

📝 Checklist

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

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[pi0]

Thanks!