Add GitLabDemoUpstream.jl as a dependency

Just following https://docs.gitlab.com/ee/ci/ssh_keys/README.html

Concrete steps:

1. Create an SSH key pair with ssh-keygen -t ed25519 -C GitLabDemoDownstream@gitlab-ci -f key
2. Add the public key as a deploy key at https://gitlab.com/tkfm/gitlabdemoupstream.jl/-/settings/repository#js-deploy-keys-settings
3. Add the private key as SSH_PRIVATE_KEY environment variable at https://gitlab.com/tkfm/gitlabdemodownstream.jl/-/settings/ci_cd
   • Turn off Protected switch.
     • Without this, $SSH_PRIVATE_KEY is not set and ssh-add would complain that Error loading key "(stdin)": invalid format https://gitlab.com/tkfm/gitlabdemodownstream.jl/-/jobs/759713949#L107