arcade: Arcade SourceLink Failures

Looks like something changed in Arcade.SDK recently and is causing source link validation to fail:

Validating Microsoft.DotNet.Arcade.Sdk.1.0.0-beta.19279.1.symbols.nupkg ...
        File tools/net472/Microsoft.DotNet.Arcade.Sdk.dll has broken links:
                Failed to retrieve https://raw.githubusercontent.com/dotnet/arcade/ea6addfdc65e5df1b2c036f11614a5f922e36267/.packages/microsoft.bcl.json.sources/4.6.0-preview4.19202.2/contentFiles/cs/netstandard2.0/Microsoft.Bcl.Json.Sources/SR.cs (700)
                Failed to retrieve https://raw.githubusercontent.com/dotnet/arcade/ea6addfdc65e5df1b2c036f11614a5f922e36267/.packages/microsoft.bcl.json.sources/4.6.0-preview4.19202.2/contentFiles/cs/netstandard2.0/Microsoft.Bcl.Json.Sources/System.SR.cs (700)
                Failed to retrieve https://raw.githubusercontent.com/dotnet/arcade/ea6addfdc65e5df1b2c036f11614a5f922e36267/.packages/microsoft.bcl.json.sources/4.6.0-preview4.19202.2/contentFiles/cs/netstandard2.0/Microsoft.Bcl.Json.Sources/BitStack.cs (700)
....

The XUnitConsoleRunner package is also failing the validation (this one seems to be a problem related to git submodules + sourcelink links):

Validating Microsoft.DotNet.XUnitConsoleRunner.2.5.1-beta.19279.1.symbols.nupkg ...
        File tools/netcoreapp2.1/xunit.core.dll has broken links:
                Failed to retrieve https://raw.githubusercontent.com/xunit/xunit/ad20ed120a3653d6af533b76cf8a04f096095349/src/xunit.assert/Asserts/Sdk/ArgumentFormatter.cs (700)
                Failed to retrieve https://raw.githubusercontent.com/xunit/xunit/ad20ed120a3653d6af533b76cf8a04f096095349/src/xunit.assert/Asserts/Sdk/AssertEqualityComparer.cs (700)
                Failed to retrieve https://raw.githubusercontent.com/xunit/xunit/ad20ed120a3653d6af533b76cf8a04f096095349/src/xunit.assert/Asserts/Sdk/AssertEqualityComparerAdapter.cs (700)

@tmat @ViktorHofer