[WinUI OSS] Automate shine-oss availability of pinned .NET 8 targeting packs

### Description

Fresh public clones of  microsoft/microsoft-ui-xaml  intermittently fail to restore with  HTTP 401 Unauthorized  on  Microsoft.NETCore.App.Ref ,  Microsoft.WindowsDesktop.App.Ref  and  Microsoft.AspNetCore.App.Ref , after repeated sign-in prompts.

#### Root cause:  
build\DownloadDotNetCoreSdk.ps1  installs the repo-local SDK by channel ( dotNetSdkChannel  =  9.0.3xx ), not by exact version, so the .NET 8 targeting packs the SDK implicitly requests drift with the calendar (measured: SDK 9.0.316 → 8.0.29, SDK 9.0.317 → 8.0.30). The shine-oss  @Local  view only contains versions that were previously fetched while authenticated; Azure Artifacts does not let anonymous users trigger an upstream save. Any version not already saved therefore 401s for external contributors.

PR "Pin .NET 8 targeting packs to 8.0.28 so OSS clones can restore" fixes the immediate outage by pinning to a version already present in  @Local . It does not prevent recurrence: the next time the pin is raised to a version nobody has fetched while authenticated, public clones break again.