Paket 7 in multi-target repo fails with: 'Windows is required to build Windows desktop applications.'

Description

In a solution with separate macOS and Windows Desktop projects, paket install now fails with errors such as:

/usr/local/share/dotnet/sdk/6.0.300/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(90,5): error NETSDK1100: Windows is required to build Windows desktop applications. [/Users/jwostenberg/Code/Interstellar/src/Interstellar.Wpf.Chromium/Interstellar.Wpf.Chromium.fsproj]
/usr/local/share/dotnet/sdk/6.0.300/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(90,5): error NETSDK1100: Windows is required to build Windows desktop applications. [/Users/jwostenberg/Code/Interstellar/Examples/Examples.Wpf.Chromium/Examples.Wpf.Chromium.fsproj]
/usr/local/share/dotnet/sdk/6.0.300/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(90,5): error NETSDK1100: Windows is required to build Windows desktop applications. [/Users/jwostenberg/Code/Interstellar/src/Interstellar.WinForms.Chromium/Interstellar.WinForms.Chromium.fsproj]
/usr/local/share/dotnet/sdk/6.0.300/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(90,5): error NETSDK1100: Windows is required to build Windows desktop applications. [/Users/jwostenberg/Code/Interstellar/Examples/Examples.WinForms.Chromium/Examples.WinForms.Chromium.fsproj]
Total time taken: 10 milliseconds
Paket failed with
-> dotnet restore "Interstellar.Windows.sln" failed

Repro steps

1. Have a repo with two solutions and two projects
   1a. MyProject.Windows.sln which contains MyProject.Windows.fsproj with a TFM of net6.0-windows
   1b. MyProject.macOS.sln which contains MyProject.macOS.fsproj with a TFM of net6.0-macos
2. On macOS, try to do a dotnet paket install

Expected behavior

paket install should work and not fail

Actual behavior

Paket blows up

Known workarounds

Downgrade to Paket 6.0.13.

Reproduce the failure on macOS with the described Windows and macOS solutions by running dotnet paket install, then compare the behavior with Paket 6.0.13. Trace Paket's restore handling for multi-target solutions and the listed .sln and .fsproj files. Done means Paket 7 completes installation without requiring Windows for the Windows-targeted projects.