build: properly ignore xamarin+winui targets for shadows (backport #762)

[mergify]

This is an automatic backport of pull request #762 done by Mergify.
Cherry-pick of 0812002 has failed:

On branch mergify/bp/release/stable/3.1/pr-762
Your branch is up to date with 'origin/release/stable/3.1'.

You are currently cherry-picking commit 0812002.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   src/Uno.Toolkit.RuntimeTests/Tests/ShadowContainerTests.cs
	deleted by us:   src/Uno.Toolkit.Skia.WinUI/Controls/Shadows/ShadowContainer.cs
	deleted by us:   src/Uno.Toolkit.Skia.WinUI/Uno.Toolkit.Skia.WinUI.csproj

no changes added to commit (use "git add" and/or "git commit -a")

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[agneszitte]

@kazo0 this one needs fixes please