Current WCT solution does not build when .NET 5 preview is installed

Describe the bug

When building the Microsoft.Toolkit.Services project, the WPFPopup XAML page for the net462 target is not getting its .i.g.cs content generated properly, causing missing members compilation errors.

Steps to Reproduce

Install .NET 5.0.100-preview.6.20277.5, build the solution.

Additional context

This looks like a .NET 5 issue, where the MarkupCompilePass1 does not seem to be executed at all, even though Page ItemGroup items are present.

As a workaround, adding this to the global.json file works around the issue:

  "sdk": {
    "version": "3.1.201",
    "rollForward": "latestPatch"
  },