Using ssh.Unmarshal since commit c11ed6a015a8da means the "shell" payload should be propely formatted, with a specified string length. Otherwise, when attempting to connect, you'll see this message in the gokrazy stderr:
ssh: parse error in message type 0
that's because in x/crypto/ssh, the parseString() method expects a length before the string. If length is 0, there will be remaining unparsed data, and an error.
This may still result in an error if sh is not present, but in case sh *is*
present, it results in the user just getting a shell without having to use
e.g. ssh -t gokrazy ./busybox sh
