Include NuGet.config in Helix test payloads

[JoeRobich]

Fixes a hermeticity gap in Roslyn Helix test work items.

PrepareTests preserves the repository NuGet.config, but HelixTestRunner only included eng and global.json in each generated work-item payload. As a result, runtime NuGet clients such as Microsoft.CodeAnalysis.Testing.ReferenceAssemblies loaded the Helix machine defaults and attempted to resolve packages from nuget.org.

This change includes NuGet.config at every work-item root. The config clears inherited package sources and uses the approved feeds, including dotnet-public.

This addresses the Microsoft.NETCore.App.Ref.10.0.1 failures observed in build 1587731 for #85107.

Validation:

• Built src/Tools/RunTests/RunTests.csproj
• Generated 27 local Helix work-item payloads and verified each contained NuGet.config
• Ran the exact failing SkippedNamespace_MoreDerivedNamespace test with fresh NuGet and temporary package caches; it resolved Microsoft.NETCore.App.Ref.10.0.1 successfully
• Ran all Microsoft.CodeAnalysis.PublicApiAnalyzers.UnitTests: 317 passed

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🟢 Approval recommended

The change is a small, targeted update consistent with existing payload setup patterns and directly addresses the missing config in work-item roots.

Review tier: Lite
Findings: None

What changed in this PR

Updates the Helix work-item payload generation in RunTests so each work item includes the repository NuGet.config, ensuring NuGet restore/asset resolution in Helix runs uses the repo-approved feeds rather than machine defaults.

Changes:

• Add NuGet.config to each generated Helix work-item payload (alongside existing eng and global.json).
• Preserve the existing payload-shaping approach by using a symbolic link from the work-item root to the repo root NuGet.config.

File	Description
src/​Tools/​RunTests/​HelixTestRunner.cs	Adds a NuGet.config symlink into each work-item payload directory so Helix executions pick up repo feed configuration.