Switch from ctaggart's SourceLink to dotnet's

ctaggart's SourceLink has at some point started packing `.pdb` files
into the main Nuget package, which is not ideal if we want to publish
a separate `.snupkg` to NuGet.

For that reason, let's make the switch to `dotnet/SourceLink` even
though it is *still* only in beta.

References:
 * ctaggart/SourceLink#291

build/SourceLink.props

@@ -2,11 +2,11 @@
 
 	<PropertyGroup>
 		<MSBuildAllProjects>$(MSBuildAllProjects);$(MSBuildThisFileFullPath)</MSBuildAllProjects>
-		<SourceLinkCreate Condition="'$(BuildingInsideVisualStudio)' != 'True'">True</SourceLinkCreate>
+		<PublishRepositoryUrl>True</PublishRepositoryUrl>
 	</PropertyGroup>
 
 	<ItemGroup>
-		<PackageReference Include="SourceLink.Create.CommandLine" Version="2.7.3" PrivateAssets="All" />
+		<PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-18618-05" PrivateAssets="All"/>
 	</ItemGroup>
 
 </Project>