Mark Microsoft.NET.ILLink as non-shipping (#82905)

This package should not be published to nuget. It is only used by xamarin, and contains the API surface for custom illink steps.

Author: sbomer

src/tools/illink/src/linker/Mono.Linker.csproj

@@ -7,6 +7,7 @@
     <Description>IL Linker</Description>
     <RootNamespace>Mono.Linker</RootNamespace>
     <IsPackable>true</IsPackable>
+    <IsShipping>false</IsShipping>
     <PackageId>Microsoft.NET.ILLink</PackageId>
     <!-- Disabling baseline validation since this is a brand new package.
        Once this package has shipped a stable version, the following line