tee: slower than GNU

Using GNU `yes` from master (vmsplice) and main `tee`
```
$ gnu58d88d243/yes | gnu58d88d243/tee | pv>/dev/null
[2.07GiB/s]
$ gnu58d88d243/yes | uu-main-tee | pv>/dev/null
[1.93GiB/s]
```