Fix CI deploy release job

The CI deploy_release job no longer worked, because git.astron.nl no longer accepts pushes from unverified accounts using https. This MR fixes the deploy_release job for the current pipeline.

It uses the protected variable (see "Settings | CI/CD | Variables") SSH_PRIVATE_KEY that contains the private part of an SSH key-pair. The public part is stored as deploy key (see "Settings | Repository | Deploy Keys").