`dotnet workload restore` fails on macOS when a project uses an SDK that is not yet in the package cache

[AArnott]

Describe the bug

When dotnet workload restore is run in a directory containing a project that requires an SDK to be restored, it fails.

Interestingly, it only fails on my macOS agent. Linux and Windows appear to have no problem with it.

To Reproduce

Create a directory with the following dirs.proj in it:

<Project Sdk="Microsoft.Build.Traversal">
  <ItemGroup>
  </ItemGroup>
</Project>

I also had this global.json in place:

{
	"sdk": {
		"version": "8.0.400",
		"rollForward": "patch",
		"allowPrerelease": false
	},
	"msbuild-sdks": {
		"Microsoft.Build.Traversal": "4.1.0"
	}
}

Set the environment variable NUGET_PACKAGES to an empty or non-existent directory.

Run dotnet workload restore

Expected

A successful workload restore

Actual

This failure:

Unhandled exception: Microsoft.Build.Exceptions.InvalidProjectFileException: The SDK 'Microsoft.Build.Traversal' specified could not be found.  /Users/buildagent/myagent/_work/1/s/azure-pipelines/dirs.proj
   at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject(String errorSubCategoryResourceName, IElementLocation elementLocation, String resourceName, Object[] args)
   at Microsoft.Build.Shared.ProjectErrorUtilities.ThrowInvalidProject[T1](IElementLocation elementLocation, String resourceName, T1 arg0)
   at Microsoft.Build.Evaluation.Evaluator`4.ExpandAndLoadImportsFromUnescapedImportExpressionConditioned(String directoryOfImportingFile, ProjectImportElement importElement, List`1& projects, SdkResult& sdkResult)
   at Microsoft.Build.Evaluation.Evaluator`4.ExpandAndLoadImports(String directoryOfImportingFile, ProjectImportElement importElement, SdkResult& sdkResult)
   at Microsoft.Build.Evaluation.Evaluator`4.EvaluateImportElement(String directoryOfImportingFile, ProjectImportElement importElement)
   at Microsoft.Build.Evaluation.Evaluator`4.PerformDepthFirstPass(ProjectRootElement currentProjectOrImport)
   at Microsoft.Build.Evaluation.Evaluator`4.Evaluate()
   at Microsoft.Build.Evaluation.Evaluator`4.Evaluate(IEvaluatorData`4 data, Project project, ProjectRootElement root, ProjectLoadSettings loadSettings, Int32 maxNodeCount, PropertyDictionary`1 environmentProperties, ILoggingService loggingService, IItemFactory`2 itemFactory, IToolsetProvider toolsetProvider, IDirectoryCacheFactory directoryCacheFactory, ProjectRootElementCacheBase projectRootElementCache, BuildEventContext buildEventContext, ISdkResolverService sdkResolverService, Int32 submissionId, EvaluationContext evaluationContext, Boolean interactive)
   at Microsoft.Build.Execution.ProjectInstance.Initialize(ProjectRootElement xml, IDictionary`2 globalProperties, String explicitToolsVersion, String explicitSubToolsetVersion, Int32 visualStudioVersionFromSolution, BuildParameters buildParameters, ILoggingService loggingService, BuildEventContext buildEventContext, ISdkResolverService sdkResolverService, Int32 submissionId, Nullable`1 projectLoadSettings, EvaluationContext evaluationContext, IDirectoryCacheFactory directoryCacheFactory)
   at Microsoft.Build.Execution.ProjectInstance..ctor(String projectFile, IDictionary`2 globalProperties, String toolsVersion, String subToolsetVersion, ProjectCollection projectCollection, Nullable`1 projectLoadSettings, EvaluationContext evaluationContext, IDirectoryCacheFactory directoryCacheFactory, Boolean interactive)
   at Microsoft.Build.Execution.ProjectInstance..ctor(String projectFile, IDictionary`2 globalProperties, String toolsVersion)
   at Microsoft.DotNet.Workloads.Workload.Restore.WorkloadRestoreCommand.RunTargetToGetWorkloadIds(IEnumerable`1 allProjects)
   at Microsoft.DotNet.Workloads.Workload.Restore.WorkloadRestoreCommand.Execute()
   at System.CommandLine.Invocation.InvocationPipeline.Invoke(ParseResult parseResult)
   at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime, ITelemetry telemetryClient)

Workaround

Running dotnet restore before dotnet workload restore avoids the problem.