[One .NET] properly set $(UseAppHost) to false

[jonathanpeppers]

Context: dotnet/sdk#19891
Context: https://github.com/xamarin/xamarin-macios/blob/a6eb528197854c074d9dd5847328c096890337be/dotnet/targets/Xamarin.Shared.Sdk.props#L38-L52

We currently have a small hack in place to prevent Linux packages from
being used in .NET 6 builds:

<!-- HACK: workaround for: https://github.com/dotnet/sdk/issues/19891 -->
<Target Name="_RemoveLinuxFrameworkReferences"
    AfterTargets="ProcessFrameworkReferences">
  <ItemGroup>
    <_ProblematicRIDs Include="linux-arm;linux-arm64;linux-x86;linux-x64" />
    <PackageDownload Remove="Microsoft.AspNetCore.App.Runtime.%(_ProblematicRIDs.Identity)" />
    <PackageDownload Remove="Microsoft.NETCore.App.Host.%(_ProblematicRIDs.Identity)" />
  </ItemGroup>
</Target>

The Xamarin.iOS team found if you set this late during
$(AfterMicrosoftNETSdkTargets):

<_RuntimeIdentifierUsesAppHost>false</_RuntimeIdentifierUsesAppHost>
<UseAppHost>false</UseAppHost>

Then Microsoft.NETCore.App.Host.* packs won't be brought in.

After making this change, I can remove this line from the workaround:

<PackageDownload Remove="Microsoft.NETCore.App.Host.%(_ProblematicRIDs.Identity)" />

The final solution for dotnet/sdk#19891, will be a future change that
makes Microsoft.AspNetCore.App.Runtime have a way to ignore
unsupported platforms.

[jonathanpeppers]

We can ignore the 1 test failure here:

[BT : 1.8.1] error : Error retrieving device SDK version. Please try again. [/Users/runner/work/1/s/bin/TestRelease/temp/TestAndroidStoreKeyTrueTrueaabTrueandroid_ks test.keystoreTrue/UnnamedProject.csproj]
  /Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(2516,3): error : java.lang.IllegalStateException :  Error retrieving device SDK version. Please try again. [/Users/runner/work/1/s/bin/TestRelease/temp/TestAndroidStoreKeyTrueTrueaabTrueandroid_ks test.keystoreTrue/UnnamedProject.csproj]

    0 Warning(s)
    2 Error(s)