synchronize exec combined output buffer when necessary

[BenTheElder]

When we execute things in kind we capture the output in case of an error and provide this when debugging. This is very handy, but had a nasty bug, xref: #971.

When cmd.Stdout == cmd.Stderr, go will set up a single fd passed to the underyling process, and single io.Copy goroutine will be used for that fd. This makes it safe to share a buffer between cmd.Stdout and cmd.Stderr IFF cmd.Stdout == cmd.Stderr.

When this is not the case, we need to synchronize writes to the combined output buffer.

See:

• https://go-review.googlesource.com/c/go/+/42052/ (os/exec: document that non-comparable writers may race)
• https://github.com/golang/go/blob/72275c0dea7887597e3e445676674ba6af702f06/src/os/exec/exec.go#L216-L223
• https://github.com/golang/go/blob/72275c0dea7887597e3e445676674ba6af702f06/src/os/exec/exec.go#L282-L287

fixes: #971

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[BenTheElder]

/priority critical-urgent
/kind bug

[BenTheElder]

IPv6 e2e flakes
/test pull-kind-conformance-parallel-ipv6

[aojea]

nice catch and thanks to @liggitt for keep finding occurrences
/lgtm

the ipv6 flakes show errors with some application holding the lock, I'm trying to investigate more

[BenTheElder]

the ipv6 flakes show errors with some application holding the lock, I'm trying to investigate more

[iptables, on the node, not the lock we introduced here]