Adding explicit PackageReference for System.Runtime.CompilerServices.Unsafe.

Forge.TreeWalker.UnitTests/Forge.TreeWalker.UnitTests.csproj

@@ -39,6 +39,7 @@
     <PackageReference Include="NuGet.Build.Tasks.Pack" Version="5.2.0">
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.Scripting" Version="3.8.0" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="Newtonsoft.Json.Schema" Version="3.0.15" />