Only add package on non-windows os

Author: RehanSaeed

Source/Schema.NET/Schema.NET.csproj

@@ -43,7 +43,7 @@
   <ItemGroup Label="Package References">
     <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" PrivateAssets="all" Version="2.9.2" />
     <!-- See https://github.com/Microsoft/dotnet/tree/master/releases/reference-assemblies -->
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0-preview.2" />
+    <PackageReference Condition="'$(OS)' != 'Windows_NT'" Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0-preview.2" />
     <PackageReference Condition="'$(OS)' == 'Windows_NT'" Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" Version="1.0.0-beta2-19270-01" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
     <PackageReference Include="StyleCop.Analyzers" PrivateAssets="All" Version="1.1.118" />