fix: resolve output dirs relative to rootDir

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#22038

❓ 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

Debugging another issue (nuxt/nuxt#22038) noticed that output dirs were being resolved relative to srcDir in the absence of an explicit base.

📝 Checklist

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

[pi0]

💯 (waiting for ci to pass)

[codecov]

Codecov Report

Merging #1666 (625c621) into main (19916e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1666   +/-   ##
=======================================
  Coverage   77.77%   77.78%           
=======================================
  Files          76       76           
  Lines        7824     7827    +3     
  Branches      803      804    +1     
=======================================
+ Hits         6085     6088    +3     
  Misses       1737     1737           
  Partials        2        2           

Files Changed	Coverage Δ
src/options.ts	96.99% <100.00%> (+0.01%)	⬆️