This fixes SSH_INSCRIBE_USE_AGENT env var to act as boolean instead of only value 0 disabling it.
In addition fixes all other boolean env vars (SSH_INSCRIBE_WRITE, SSH_INSCRIBE_RENEW, SSH_INSCRIBE_GENKEY, SSH_INSCRIBE_DEBUG, SSH_INSCRIBE_INSECURE, SSH_INSCRIBE_QUIET) to act as actual booleans instead of non-empty value setting the config option to true.
This fixes SSH_INSCRIBE_USE_AGENT env var to act as boolean instead of only value 0 disabling it.
In addition fixes all other boolean env vars (SSH_INSCRIBE_WRITE, SSH_INSCRIBE_RENEW, SSH_INSCRIBE_GENKEY, SSH_INSCRIBE_DEBUG, SSH_INSCRIBE_INSECURE, SSH_INSCRIBE_QUIET) to act as actual booleans instead of non-empty value setting the config option to true.