fix(firebase-app-hosting): use relative path in runCommand

[Yuangwang]

🔗 Linked issue

Discussed on discord, needed to support frameworks that change the location of serverDir

(note by @pi0): this is a temporary solution to support AnalogJS until it changes back to nitro defaults in next major versions.

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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

This addresses and issue where the hard coded runCommand doesn't work if the server directory location is changed

📝 Checklist

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

[pi0]

Thanks.