Speed up CI Windows metadata setup

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• replace the hosted-runner Visual Studio UAP SDK install with NuGet-provided Windows metadata for CsWinRT
• add workspace-scoped NuGet caching to the test and release workflows
• remove stale commented-out lines from the test workflow

Validation

• local: dotnet restore src/UniGetUI.sln
• local: dotnet test src/UniGetUI.sln --no-restore --verbosity q --nologo
• GitHub Actions dry run after rebase: https://github.com/Devolutions/UniGetUI/actions/runs/22974519179

Notes

• rebase onto main completed cleanly with no conflicts
• dry run succeeded for preflight, both build matrix jobs, and publish-in-dry-run flow