In order to switch the NixOS configuration, we must have root access.
If we are connecting to the target host with a non-root user, we use the "sudo" command.
We could instead use `nixos-rebuild --use-remote-sudo --target-host` but it
would evaluate the nixos system again.
Only one callback can be installed per signal in Bash. Because the
previous `trap` removes the whole temporary workdir, the one around the
SSH key is redundant.
`nixos-rebuild/deploy.sh` script enable ssh authentication with a given private key
through the `SSH_KEY` environment variable.
Add additional variable for the private key used for the deployment.
To encourage the use of ssh-agent and discourage the storage of deployment keys in the terraform state
we do not set the install ssh key as the default for the deployment key.
Co-authored-by: Jörg Thalheim <Mic92@users.noreply.github.com>
