Guard background AST API for CoreCLR

Co-authored-by: bergmeister <9250262+bergmeister@users.noreply.github.com>

Author: Copilot

Rules/AvoidAlias.cs

@@ -318,7 +318,9 @@ private ScriptBlockExpressionAst GetDscResourceBody(CommandAst cmdAst)
             if (!(cmdAst.Parent is PipelineAst commandPipeline)
                 || commandPipeline.PipelineElements.Count != 1
                 || commandPipeline.PipelineElements[0] != cmdAst
+#if CORECLR
                 || commandPipeline.Background
+#endif
                 || !(commandPipeline.Parent is NamedBlockAst || commandPipeline.Parent is StatementBlockAst))
             {
                 return null;
@@ -347,7 +349,9 @@ private ScriptBlockExpressionAst GetDscResourceBody(CommandAst cmdAst)
                     if (index >= 0 && index + 1 < statements.Count
                         && statements[index + 1] is PipelineAst bodyPipeline
                         && bodyPipeline.PipelineElements.Count == 1
+#if CORECLR
                         && !bodyPipeline.Background
+#endif
                         && bodyPipeline.PipelineElements[0] is CommandExpressionAst bodyExpression)
                     {
                         if (HasStatementSeparator(cmdAst, bodyPipeline)