Restore task failing on our solutions

[Anipik]

Expected behavior

C:\Users\anagniho\source\repos\ConsoleApp10>dotnet build ConsoleApp10.sln
Microsoft (R) Build Engine version 16.7.0-preview-20270-03+bee129d1b for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  Restored C:\Users\anagniho\source\repos\ConsoleApp10\ConsoleApp10.csproj (in 345 ms).
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  ConsoleApp10 -> C:\Users\anagniho\source\repos\ConsoleApp10\bin\Debug\netcoreapp3.1\ConsoleApp10.dll

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:03.34

Actual Behavior

C:\git\runtime\src\libraries\System.Diagnostics.TraceSource>dotnet build
Microsoft (R) Build Engine version 16.7.0-preview-20270-03+bee129d1b for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
C:\Program Files\dotnet\sdk\5.0.100-preview.6.20301.7\NuGet.targets(128,5): error : Object reference not set to an instance of an object. [C:\git\runtime\src\libraries\System.Diagnostics.TraceSource\System.Diagnostics.TraceSource.sln]

Build FAILED.

cc @ericstj @ViktorHofer @safern

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Tagging subscribers to this area: @safern, @ViktorHofer
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[ViktorHofer]

Spoke with @jeffkl. Static graph restore currently doesn't work on slns. We probably need to bring back this piece of code in the TargetFramework SDK: dotnet/arcade@4a290d3#diff-e3462a51d4395b12c22c7012edc37a3fL43-L59

EDIT: it works, the solution simply didn't honor the static graph restore feature switch. See linked PR.