We could default to ~/.ssh/id_rsa but if that doesn't exist, then we'll
just be confusing people in the logs. Best is to just rely on the
ssh-agent which overrides this anyway. We will document this.
See coop-cloud/organising#277
This is part of trying to debug:
coop-cloud/organising#250
And also part of:
coop-cloud/docs.coopcloud.tech#27
Where I now try to specify the same logic as `ssh -i <my-key-path>` in
the underlying connection logic. This should help with being more
explicit about what key is being used via the SSH config file.