fix: add envPrefix to runtimeConfig.nitro types

[danielroe]

🔗 Linked issue

❓ 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

This adds a missing type for runtimeConfig.nitro - apologies!

(This is not a blocker for Nuxt.)

📝 Checklist

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

[codecov]

Codecov Report

Merging #1336 (18ec7bb) into main (21a2595) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1336   +/-   ##
=======================================
  Coverage   76.67%   76.67%           
=======================================
  Files          70       70           
  Lines        7160     7161    +1     
  Branches      710      710           
=======================================
+ Hits         5490     5491    +1     
  Misses       1669     1669           
  Partials        1        1           

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