fix: Fix NU1009 reference assembly warning

Author: austindrenski

Diff for: Directory.Packages.props

@@ -7,7 +7,6 @@
   <ItemGroup Label="src">
     <PackageVersion Include="Microsoft.Bcl.AsyncInterfaces" Version="8.0.0" />
     <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="2.0.0" />
-    <PackageVersion Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" />
     <PackageVersion Include="System.Collections.Immutable" Version="1.7.1" />
     <PackageVersion Include="System.Threading.Channels" Version="6.0.0" />
   </ItemGroup>
@@ -29,4 +28,8 @@
     <PackageVersion Include="xunit.runner.visualstudio" Version="[2.4.3, 3.0)" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(OS)' == 'Unix'">
+    <PackageVersion Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" />
+  </ItemGroup>
+
 </Project>