Add switch to disable apphost pack restore

Contributes to dotnet/runtime#58109

In dotnet/runtime we live build targeting packs, runtime packs and app host packs. For the former two, switches already exist to disable nuget restore:
- EnableTargetingPackDownload
- EnableRuntimePackDownload

The latter one doesn't yet have such a switch and therefore is always restored by NuGet. In the cases where want to use the live built apphost pack, restore fails, i.e. from a runtime build:

`artifacts/bin/trimmingTests/projects/Microsoft.Extensions.DependencyInjection.TrimmingTests/ActivatorUtilitiesTests/osx-x64/project.csproj(0,0): error NU1102: (NETCORE_ENGINEERING_TELEMETRY=Build) Unable to find package Microsoft.NETCore.App.Host.osx-x64 with version (= 8.0.0)`

Author: ViktorHofer

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets

@@ -146,7 +146,7 @@ Copyright (c) .NET Foundation. All rights reserved.
                      DotNetIjwHostLibraryNameWithoutExtension="$(_DotNetIjwHostLibraryNameWithoutExtension)"
                      RuntimeGraphPath="$(BundledRuntimeIdentifierGraphFile)"
                      KnownAppHostPacks="@(KnownAppHostPack)"
-                     NuGetRestoreSupported="$(_NuGetRestoreSupported)"
+                     NuGetRestoreSupported="$([MSBuild]::ValueOrDefault('$(EnableAppHostPackDownload)', '$(_NuGetRestoreSupported)'))"
                      NetCoreTargetingPackRoot="$(NetCoreTargetingPackRoot)">
 
       <Output TaskParameter="PackagesToDownload" ItemName="_PackageToDownload" />