fix: resolve server assets dir relative to srcDir

[danielroe]

🔗 Linked issue

nuxt/nuxt#18587

❓ 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

We are now resolving server assets dirs before this point (#825), when options are resolved. We probably still need to resolve server dirs when adding them after this point.

📝 Checklist

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

[codecov]

Codecov Report

Merging #893 (dc7c56b) into main (917c7e6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #893   +/-   ##
=======================================
  Coverage   67.47%   67.47%           
=======================================
  Files          59       59           
  Lines        5860     5860           
  Branches      635      637    +2     
=======================================
  Hits         3954     3954           
  Misses       1897     1897           
  Partials        9        9           

Impacted Files	Coverage Δ
src/nitro.ts	89.21% <100.00%> (ø)

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[pi0]

Thanks!