Fix "Failed tcsetattr(TCSADRAIN)" when `nix repl` is not a TTY

[9999years]

Fixes #9941.

Before:

$ echo builtins.nixVersion | nix repl
Welcome to Nix 2.18.1. Type :? for help.

Failed tcsetattr(TCSADRAIN): Inappropriate ioctl for device
"2.18.1"

Failed tcsetattr(TCSADRAIN): Inappropriate ioctl for device

After:

$ echo builtins.nixVersion | nix repl
Nix 2.21.0pre20240131_dirty
Type :? for help.
"2.21.0pre20240131_dirty"