Ignore user-supplied fd redirections above 2 for builtins

Prevents e.g. specifying an fd which corresponds to the history file
as the stdin for builtin_source

parse_execution.cpp

@@ -1083,7 +1083,7 @@ bool parse_execution_context_t::determine_io_chain(const parse_node_t &statement
                     }
                     else
                     {
-                        new_io.reset(new io_fd_t(source_fd, old_fd));
+                        new_io.reset(new io_fd_t(source_fd, old_fd, true));
                     }
                 }
                 break;