Fix for issue 19428

[sosssego]

Expanding the environmental variables before path combine

This allows %HOMEPATH% to be expanded to \Users\myuser before adding the system drive.

PR Checklist

• [x] Closes: #19428
• [ ] Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• [x] Tests: Added/updated and all pass
• [ ] Localization: All end user facing strings can be localized
• [ ] Dev docs: Added/updated
• [ ] New binaries: Added on the required places
  • [ ] JSON for signing for new binaries
  • [ ] WXS for installer for new binaries and localization folder
  • [ ] YML for CI pipeline for new test projects
  • [ ] YML for signed pipeline
• [ ] Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Manual test and added unit test