Fix line break regression

In commit d1d94cd, we accidentally introduced a regression
since conhost requires all line breaks to be `\r\n`.

src/Spectre.Console/Cli/Internal/Composer.cs

@@ -71,7 +71,7 @@ namespace Spectre.Console.Cli
         {
             for (var i = 0; i < count; i++)
             {
-                _content.Append('\n');
+                _content.Append(Environment.NewLine);
             }
 
             return this;