internal/coordinator/remote: fix log message in legacyIncomingSSHPostAuth and IncomingSSHPostAuth
internal/coordinator/remote: fix log message in legacyIncomingSSHPostAuth and IncomingSSHPostAuth

In the Plan 9 case, a log message is printed when ipErr != nil,
but the wrong variable err was displayed instead of ipErr.

Updates golang/go#53571

Change-Id: I4acab14f830b5cc0ae7a7863199c0b0f70af6524
Reviewed-on: https://go-review.googlesource.com/c/build/+/414595
Auto-Submit: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build