6.0 SDK diff: Allow dotnet/sdk to produce reference assemblies

[dagood]

dagood#9 (comment)

The 6.0.100 SDK is missing some ref/*.dll files:

./sdk/6.0.100/DotnetTools/dotnet-watch/6.0.100-rtm.21522.10/tools/net6.0/any/ref/dotnet-watch.dll
./sdk/6.0.100/Microsoft/Microsoft.NET.Build.Extensions/tools/net6.0/ref/Microsoft.NET.Build.Extensions.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk.BlazorWebAssembly/tools/net6.0/ref/Microsoft.NET.Sdk.BlazorWebAssembly.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk.Publish/tools/net6.0/ref/Microsoft.NET.Sdk.Publish.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk/tools/net6.0/ref/Microsoft.NET.Build.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk.Web.ProjectSystem/tools/net6.0/ref/Microsoft.NET.Sdk.Web.ProjectSystem.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk.Web/tools/net6.0/ref/Microsoft.NET.Sdk.Web.Tasks.dll
./sdk/6.0.100/Sdks/Microsoft.NET.Sdk.Worker/tools/net6.0/ref/Microsoft.NET.Sdk.Worker.Tasks.dll

This hasn't seemed to have any effect on SDK usage, but we aren't sure what these files actually do in the Microsoft-built SDK.

The reason they aren't being produced is an intentional source-build change in the sdk.proj:

Hmm. This was done intentionally in source-build's repos/sdk.proj:

    <!-- The sdk repo is building with an older version of the SDK where this property is
         defaulted to false.  The SDK that source-build is using defaults it to true.
         Set it to false to get old behavior for building the sdk repo. -->
    <BuildCommandArgs>$(BuildCommandArgs) /p:ProduceReferenceAssembly=false</BuildCommandArgs>

I suppose dotnet/sdk updated to a newer version of the .NET SDK as stage 0 since this comment was written, so the Microsoft build started using the new default. Then this line suddenly introduced diffs rather than fixing them. 😄

It should be simple enough to remove these lines to reduce the SDK diff.