Update PetaPoco.cs

Sql() class should reset build cache when new Append operations are called.  If not then stepping through with the debugger or doing internal logging can cause invalid SQL and unexpected results.

src/Umbraco.Core/Persistence/PetaPoco.cs

@@ -2417,6 +2417,7 @@ namespace Umbraco.Core.Persistence
 			else
 				_rhs = sql;
 
+			_sqlFinal = null;
 			return this;
 		}