Fix CI ("checkout key has zero length")

[hacklschorsch]

The CI for this project is failing:

https://app.circleci.com/pipelines/github/PrivateStorageio/PaymentServer/430/workflows/5e0d8fe0-f608-46be-95da-7a4964cc1559/jobs/449

Warning: checkout key has zero length
Writing SSH key for checkout to "/root/.ssh/id_rsa"

[hacklschorsch]

@btlogy Let's fix this together in our upcoming dev-ops pairing!

[hacklschorsch]

Maybe like this? I forgot what we did last time. Wasn't it a strange dance with CircleCI follow and unfollow the repository while being organization owner in GitHub?