cli: fix reading of client tunnel commands (#177919)

.read_line() appends to the string, so this caused the first command entered to always be read again

cli/src/tunnels/singleton_client.rs

@@ -50,6 +50,8 @@ pub async fn start_singleton_client(args: SingletonClientArgs) -> bool {
 	thread::spawn(move || {
 		let mut input = String::new();
 		loop {
+			input.truncate(0);
+
 			match std::io::stdin().read_line(&mut input) {
 				Err(_) | Ok(0) => return, // EOF or not a tty
 				_ => {}