openssh: add version with no private key access rights check

This improves fetchgitPrivate experience, as one doesn't need anymore to copy private keys for each nixbld* user and set ssh_config accordingly. A more proper way would be to add a configuration option into ssh_config and submit the patch to the upstream, however this requires much more work -- I think this solution is good enough as this option (if added) shouldn't ever be used by an end user, and we have only one specific usecase.