Easily use custom SSH keys

[sampsyo]

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Currently, Hooknook executes git commands assuming that the user it is running as has access to the repository. We should also make it easy to provide a custom SSH key for cloning from GitHub, which the user could add as a deploy key for any relevant repos.

This would be helpful for containerization, where the user's SSH keys are unlikely to be available in the usual locations.

[sampsyo]

This will require us to generate an SSH wrapper script so we can use the GIT_SSH environment variable (because git is miserable).