Add switch to disable apphost pack restore (#29137)

* 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)`

src/Tasks/Microsoft.NET.Build.Tasks/ResolveAppHosts.cs

@@ -54,6 +54,8 @@ public class ResolveAppHosts : TaskBase
         public bool NuGetRestoreSupported { get; set; } = true;
 
         public string NetCoreTargetingPackRoot { get; set; }
+
+        public bool EnableAppHostPackDownload { get; set; } = true;
 
         [Output]
         public ITaskItem[] PackagesToDownload { get; set; }
@@ -276,7 +278,6 @@ private ITaskItem GetHostItem(string runtimeIdentifier,
                 string hostRelativePathInPackage = Path.Combine("runtimes", bestAppHostRuntimeIdentifier, "native",
                     hostNameWithoutExtension + (isExecutable ? ExecutableExtension.ForRuntimeIdentifier(bestAppHostRuntimeIdentifier) : ".dll"));
 
-
                 TaskItem appHostItem = new TaskItem(itemName);
                 string appHostPackPath = null;
                 if (!string.IsNullOrEmpty(TargetingPackRoot))
@@ -289,7 +290,7 @@ private ITaskItem GetHostItem(string runtimeIdentifier,
                     appHostItem.SetMetadata(MetadataKeys.PackageDirectory, appHostPackPath);
                     appHostItem.SetMetadata(MetadataKeys.Path, Path.Combine(appHostPackPath, hostRelativePathInPackage));
                 }
-                else
+                else if (EnableAppHostPackDownload)
                 {
                     // C++/CLI does not support package download && dedup error
                     if (!NuGetRestoreSupported && !packagesToDownload.ContainsKey(hostPackName))

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

@@ -147,6 +147,7 @@ Copyright (c) .NET Foundation. All rights reserved.
                      RuntimeGraphPath="$(BundledRuntimeIdentifierGraphFile)"
                      KnownAppHostPacks="@(KnownAppHostPack)"
                      NuGetRestoreSupported="$(_NuGetRestoreSupported)"
+                     EnableAppHostPackDownload="$(EnableAppHostPackDownload)"
                      NetCoreTargetingPackRoot="$(NetCoreTargetingPackRoot)">
 
       <Output TaskParameter="PackagesToDownload" ItemName="_PackageToDownload" />