Paket/C#: Automatically restore tools for CSharp.sln.

This is not a general fix, as we not always build the
solution file, but this should improve the DX for
local developers that use the solution file.

Author: criemen

csharp/before.CSharp.sln.targets

@@ -0,0 +1,6 @@
+<Project InitialTargets="LocalToolRestore">
+  <Target Name="LocalToolRestore" BeforeTargets="PaketRestore">
+    <Message Text="Restoring tools" Importance="High" />
+    <Exec Command="dotnet tool restore" />
+  </Target>
+</Project>