Make System.Runtime.Loader use PrivateAssets

This dependency shouldn't be passed transitively to consumers.  It is also causing warnings with .NET 9's stricter security advisory due to the runtime dependencies.

Author: jimmylewis

src/LibraryManager.Build/Microsoft.Web.LibraryManager.Build.csproj

@@ -16,7 +16,7 @@
     <PackageReference Include="Microsoft.Build.Framework" PrivateAssets="all" />
     <PackageReference Include="Microsoft.Build.Tasks.Core" PrivateAssets="all" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" PrivateAssets="all" />
-    <PackageReference Include="System.Runtime.Loader" Condition="'$(TargetFramework)' == '$(NetCoreTFM)'" />
+    <PackageReference Include="System.Runtime.Loader" Condition="'$(TargetFramework)' == '$(NetCoreTFM)'" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup>