sh: Use input_file instead of hardcoding stdin everywhere in tcsetpgrp()

shell/main.cpp

@@ -123,7 +123,7 @@ Result<int> luna_main(int argc, char** argv)
         signal(SIGTTOU, SIG_IGN);
         signal(SIGINT, sigint_handler);
 
-        tcsetpgrp(STDIN_FILENO, getpgid(0));
+        tcsetpgrp(input_file->fd(), getpgid(0));
     }
 
     TRY(init_builtins());