Make dotnet-sourcelink source-buildable

[oleksandr-didyk]

Caused by https://github.com/dotnet/sourcelink/pull/933/files#r1185258895

dotnet-sourcelink should be part of source-build as per https://github.com/dotnet/sourcelink/pull/933/files#r1194071687.

Currently, allowing the project causes several OS-specific pre-builts to get pulled in:

    <Usage Id="Microsoft.AspNetCore.App.Runtime.osx-x64" Version="8.0.0-preview.3.23177.8" />
    <Usage Id="Microsoft.AspNetCore.App.Runtime.win-x64" Version="8.0.0-preview.3.23177.8" />
    <Usage Id="Microsoft.AspNetCore.App.Runtime.win-x86" Version="8.0.0-preview.3.23177.8" />
    <Usage Id="Microsoft.NETCore.App.Host.osx-x64" Version="8.0.0-preview.3.23174.8" />
    <Usage Id="Microsoft.NETCore.App.Host.win-x64" Version="8.0.0-preview.3.23174.8" />
    <Usage Id="Microsoft.NETCore.App.Host.win-x86" Version="8.0.0-preview.3.23174.8" />
    <Usage Id="Microsoft.NETCore.App.Runtime.osx-x64" Version="8.0.0-preview.3.23174.8" />
    <Usage Id="Microsoft.NETCore.App.Runtime.win-x64" Version="8.0.0-preview.3.23174.8" />
    <Usage Id="Microsoft.NETCore.App.Runtime.win-x86" Version="8.0.0-preview.3.23174.8" />

We should test if the property responsible for them (PackAsToolShimRuntimeIdentifiers) is even needed on Linux. If it is not - condition it out to resolve the pre-builts.

[NikolaMilosavljevic]

This is required for dotnet/source-build#3052

[NikolaMilosavljevic]

Working on this.