fix(aws-lambda): join cookies with semicolon

[danielroe]

🔗 Linked issue

https://github.com/nuxt/framework/issues/5947, possibly https://github.com/nuxt/framework/issues/3803.

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [x] 🐞 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

Fixes a typo where we are joining cookies with comma rather than semicolon.

📝 Checklist

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