Port dotnet/sdk SourceBuild patches from dotnet/installer

[eerhardt]

In order to get OmniSharp working with a source-built .NET SDK, we need to build Microsoft.NET.Build.Tasks for netfx. However, we have some code that explicitly removes these assemblies from building for netfx:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj

Lines 15 to 16 in 7bffe25

	<TargetFrameworks>$(SdkTargetFramework);net472</TargetFrameworks>
	<TargetFrameworks Condition=" '$(DotNetBuildFromSource)' == 'true' ">$(SdkTargetFramework)</TargetFrameworks>

sdk/src/Tasks/Microsoft.NET.Build.Extensions.Tasks/Microsoft.NET.Build.Extensions.Tasks.csproj

Lines 18 to 19 in 7bffe25

	<TargetFrameworks>$(SdkTargetFramework);net472</TargetFrameworks>
	<TargetFrameworks Condition=" '$(DotNetBuildFromSource)' == 'true' ">$(SdkTargetFramework)</TargetFrameworks>

This causes OmniSharp to not work with a 6.0 source-built SDK.

In order to work around this for source-build, we are introducing a patch for 6.0.0. We should port these patches into the dotnet/sdk repo proper.

[sfoslund]

@eerhardt what is the timeline on this change, do we need to make it for 6.0.2xx?

[eerhardt]

I don't know what kinds of changes are accepted into servicing, but my opinion is that we should remove these SourceBuild patches as soon as we can after 6.0 is out the door. I would have made them in the 6.0.1xx branch if it wasn't so close to the release.

So my opinion is that we should take this into 6.0.1xx in the first servicing release after 6.0.100 is released.