Revert "Remove pinned package references. Now that HostModel is using…

… 6.0.0 and MSBuild has the binding redirects for SCI, we can remove these references."

This reverts commit 3ed958e.

src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj

@@ -59,6 +59,18 @@
     <ProjectReference Include="..\..\ApiCompat\Microsoft.DotNet.ApiCompat.Task\Microsoft.DotNet.ApiCompat.Task.csproj" OutputItemType="ReferenceCopyLocalPaths" ReferenceOutputAssembly="false" />
   </ItemGroup>
 
+
+  <!-- Packages that are in-box for .NET Core, so we only need to reference them for .NET Framework -->
+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework' and '$(DotNetBuildFromSource)' != 'true'">
+    <!--
+        MSBuild depends on a new version of SRM/SCI; HostModel a very old one.
+        In order to keep working with MSBuild 17.2, which had binding redirects
+        for SCI 0-5.0.0.0 but not 6, this task must compile against 5, even
+        though that's not the very latest. -->
+    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Reflection.Metadata" Version="5.0.0" />
+  </ItemGroup>
+
   <!-- These are loaded from the CLI's copy on .NET Core, we don't need to duplicate them on disk -->
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">
     <PackageReference Update="Microsoft.Extensions.DependencyModel" ExcludeAssets="Runtime" />