Offline CI legs started failing because of prebuilts

[MichaelSimons]

Sample failing CI (internal Microsoft link)

##[error]/vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj(0,0): error NU1102: (NETCORE_ENGINEERING_TELEMETRY=Restore) Unable to find package Microsoft.NETCore.App.Runtime.win-x64 with version (= 8.0.0-preview.3.23174.8)

• Found 1 version(s) in prebuilt [ Nearest version: 7.0.2 ]
• Found 0 version(s) in source-built
• Found 0 version(s) in previously-source-built
• Found 0 version(s) in reference-packages
  /vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj : error NU1102: Unable to find package Microsoft.NETCore.App.Runtime.win-x86 with version (= 8.0.0-preview.3.23174.8) [/vmr/src/sourcelink/artifacts/source-build/self/src/SourceLink.sln]
  /vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj : error NU1102: - Found 1 version(s) in prebuilt [ Nearest version: 7.0.2 ] [/vmr/src/sourcelink/artifacts/source-build/self/src/SourceLink.sln]
  /vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj : error NU1102: - Found 0 version(s) in source-built [/vmr/src/sourcelink/artifacts/source-build/self/src/SourceLink.sln]
  /vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj : error NU1102: - Found 0 version(s) in previously-source-built [/vmr/src/sourcelink/artifacts/source-build/self/src/SourceLink.sln]
  /vmr/src/sourcelink/artifacts/source-build/self/src/src/dotnet-sourcelink/dotnet-sourcelink.csproj : error NU1102: - Found 0 version(s) in reference-packages [/vmr/src/sourcelink/artifacts/source-build/self/src/SourceLink.sln]

The fix for this is contained in dotnet/sourcelink#933 where the dotnet-sourcelink.csproj is being excluded from source-build.