Cmdpal: Fix cmdpal toolkit restore failure for slnx in release pipeline

[vanzue]

Summary of the Pull Request

Error from pipeline:

Invalid input 'PowerToys.slnx'. The file type was not recognized. MSBuild version 17.14.23+b0019275e for .NET Framework Build started 12/8/2025 6:33:14 AM.

Nuget support for slnx will be ready in nuget version 7, so use msbuild to restore

PR Checklist

• [ ] Closes: #xxx

• [ ] Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• [ ] 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

Validation Steps Performed