Improve PublicSign condition

This ensures the assemblies are public signed only when they
really need to be: on linux and macOS

This means that dotnet build on Windows will now properly sign the
assemblies.

Author: bording

Directory.Build.targets

@@ -1,6 +1,5 @@
 <Project>
   <PropertyGroup>
-    <PublicSign Condition=" '$(AssemblyOriginatorKeyFile)' != '' and '$(MSBuildRuntimeType)' == 'Core' ">true</PublicSign>
+    <PublicSign Condition=" '$(AssemblyOriginatorKeyFile)' != '' and '$(OS)' != 'Windows_NT' ">true</PublicSign>
   </PropertyGroup>
 </Project>
-