message/pipeline: skip test if the 'go' executable is not found Fixes golang/go#32844 Change-Id: I9d5364df4f827db4de3c144a565c1d05d8a181bd Reviewed-on: https://go-review.googlesource.com/c/text/+/192325 Run-TryBot: Bryan C. Mills <[email protected]> TryBot-Result: Gobot Gobot <[email protected]> Reviewed-by: Alexander Rakoczy <[email protected]>