Add check for HOME with OpenSSH Windows

What does this MR do?

From customer interaction in ZD it was noted that when using OpenSSH for Windows that environment variable HOME was not being set.

This caused the SSH private keys to not be found and then prompting the user for a password when cloning via SSH.

This documentation update is to add this into the troubleshooting section for https://docs.gitlab.com/ee/user/ssh.html#password-prompt-with-git-clone

Related issues

ZD

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