[xharness] Serialize access to nuget. Fixes xamarin/maccore#1779. (#6402)

Nuget might freak out if multiple nuget processes run at the same time, so
make sure that doesn't happen.

Fixes https://github.com/xamarin/maccore/issues/1779.

tests/xharness/Jenkins.cs

@@ -72,6 +72,7 @@ namespace xharness
 		Dictionary<string, MakeTask> DependencyTasks = new Dictionary<string, MakeTask> ();
 
 		internal static Resource DesktopResource = new Resource ("Desktop", Environment.ProcessorCount);
+		internal static Resource NugetResource = new Resource ("Nuget", 1); // nuget is not parallel-safe :(
 
 		static Dictionary<string, Resource> device_resources = new Dictionary<string, Resource> ();
 		internal static Resources GetDeviceResources (IEnumerable<Device> devices)
@@ -2222,9 +2223,9 @@ namespace xharness
 						}
 					}
 
-					var resources = device_resources.Values.Concat (new Resource [] { DesktopResource });
+					var resources = device_resources.Values.Concat (new Resource [] { DesktopResource, NugetResource });
 					if (resources.Any ()) {
-						writer.WriteLine ($"<h3>Devices:</h3>");
+						writer.WriteLine ($"<h3>Devices/Resources:</h3>");
 						foreach (var dr in resources.OrderBy ((v) => v.Description, StringComparer.OrdinalIgnoreCase)) {
 							writer.WriteLine ($"{dr.Description} - {dr.Users}/{dr.MaxConcurrentUsers} users - {dr.QueuedUsers} in queue<br />");
 						}
@@ -2744,6 +2745,7 @@ namespace xharness
 
 		async Task<TestExecutingResult> RestoreNugetsAsync (string projectPath, Log log, bool useXIBuild=false)
 		{
+			using (var resource = await Jenkins.NugetResource.AcquireExclusiveAsync ()) {
 				// we do not want to use xibuild on solutions, we will have some failures with Mac Full
 				var isSolution = projectPath.EndsWith (".sln", StringComparison.Ordinal);
 				if (!File.Exists (projectPath))
@@ -2776,6 +2778,7 @@ namespace xharness
 				LogEvent (log, "Restoring nugets completed for {0} ({1}) on path {2}", TestName, Mode, projectPath);
 				return TestExecutingResult.Succeeded;
 			}
+		}
 		
 		List<string> GetNestedReferenceProjects (string csproj)
 		{