Be able to detect SSH_AUTH_SOCK and get connection by ssh agent.

[martivo]

CI/CD systems often instead of exposing env variable for private key or adding it to default location. They use ssh agent. Add possibility to support it.

For example in jenkins pipeline these ssh and git command work fine,but this tool does not.

sshagent(credentials: ['git-credentials']) {
  sh("git clone ...")
}