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Avoid error on closed pipe

fixes https://github.com/kubernetes/kubernetes/issues/57706

from @stevekuznetsov:
> If you do `echo | grep -q`, `grep` will exit when it finds the first match
> If the `echo` is still writing to stdout it fails because there's no reader on that pipe anymore
> So we always use `grep -q <<<"${content}"` now
> since that uses a FIFO
  
```release-note
NONE
```