fix: allow overriding specific build output files with aliases

[danielroe]

🔗 Linked issue

nuxt/nuxt#13986

❓ 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

In some cases, such as the linked issue (which we could resolve by adding an alias to a stub file in the case of ssr: false), it is helpful to be able to override individual #build/some-file.js aliases, and to do this we need #build to be defined last in the object.

📝 Checklist

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

[pi0]

We have dynamic virtual imports (https://nitro.unjs.io/config/#virtual) that can be used in order to override any import. (I made this feature initially for linked issue in fact)

env.alias can be used to override other aliases for any similar situation.