Use windows openssh agent by default

[contrun]

PowerShell/openssh-portable creates a ssh agent socket SSH_AUTH_SOCK=\\.\pipe\openssh-ssh-agent (see How to run ssh-add on windows? - Stack Overflow). I have tried manually specify the environment variable %SSH_AUTH_SOCK% to be \\.\pipe\openssh-ssh-agent. It worked. I think it is reasonable to set this environment variable by default, or we can provide an option to set this environment variable.