Don't pass on sourcelink as a nuget dependency (#9533)

I fixed this in the deployed FCS package already, but I forgot to add `PrivateAssets="All"` to the Sourcelink dependency for FCS, so that it's not a requirement for consumers.

Author: baronfel

fcs/FSharp.Compiler.Service/FSharp.Compiler.Service.fsproj

@@ -725,7 +725,7 @@
     <PackageReference Include="System.Reflection.Metadata" Version="1.6.0" />
     <PackageReference Include="System.Buffers" Version="4.5.0" />
     <PackageReference Include="System.Memory" Version="4.5.3" />
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
     <PackageReference Include="System.Reflection.Emit" Version="4.3.0" />