Adding addUncaughtPages ignores prefix and serializer options

[chmelevskij]

Issue Summary

If addUncaughtPages option is present in the config any of the prefix or serializer changes are ignored

To Reproduce

1. Add prefix like

mapping: {
  allSitePage: {
    sitemap: 'pages',
     prefix: '/uk',
  },
},
addUncaughtPages: true,

2. Build pages
3. Observe links in sitemap-pages.xml not having /uk prefix in the links

Any other info e.g. Why do you consider this to be a bug? What did you expect to happen instead?

Technical details:

• Gatsby Version: 3.3.0
• Node Version: v16.13.2
• OS: MacOS
• gatsby-config.js: N/A