fix(iisnode): correctly escape backslashes for port fix

[manniL]

🔗 Linked issue

nuxt/nuxt#22961

❓ 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

Uses a more general fix than #1783 based on #1697

📝 Checklist

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

[manniL]

Updated the PR to simply escape the slashes correctly.

before:

        if (process.env.PORT.startsWith('\')) {
          process.env.NITRO_UNIX_SOCKET = process.env.PORT
          delete process.env.PORT
        }
        import('./server/index.mjs');

With this PR:

        if (process.env.PORT.startsWith('\\')) {
          process.env.NITRO_UNIX_SOCKET = process.env.PORT
          delete process.env.PORT
        }
        import('./server/index.mjs');