Modifications to ActionGraph.cs to allow FASTBuild.cs to function.

ActionGraph_FASTBuild_4.15.cs

[...]
    /// <summary>
		/// Executes a list of actions.
		/// </summary>
		public static bool ExecuteActions(List<Action> ActionsToExecute, out string ExecutorName, string TargetInfoForTelemetry, bool bIsHotReload = false)
		{
			bool Result = true;
			ExecutorName = "";
			if (ActionsToExecute.Count > 0)
			{
				DateTime StartTime = DateTime.UtcNow;

				ActionExecutor Executor;
				if(FASTBuild.IsAvailable())
				{
					Executor = new FASTBuild();
				}
				else if ((XGE.IsAvailable() && BuildConfiguration.bAllowXGE) || BuildConfiguration.bXGEExport)
				{
					Executor = new XGE();
				}
[...]