Newer OpenSSH versions seem to have problems with (some?) older ssh-rsa keys,
so I figured we could switch to ed25519 and avoid any confusion regarding
ssh-rsa support.
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
