Exit status of 1 when not all of stdout is captured in pipe

See: https://github.com/DataDog/dd-trace-py/pull/2973#discussion_r742903887

riot returns as exit status of 1 when not all of the output is received a downstream piped process


```
riot run -s ... | head -n 1
echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}"
1 0

riot run -s ... | head -n 10000
echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}"
0 0
```


suuuuper weird