line: clone over ssh to fix hung deploy
Created by: jedmund
`make deploy-infra-gateway` hangs at the "Pull latest line source" task because the HTTPS clone of the public line repo prompts for credentials with no TTY attached, and waits forever.
Switching `line_repo` to the SSH form (`git@github.com:jedmund/line.git`) fixes it. This matches what `album_sort` already does on the same server (commit 239868f1) and works because nuc-mini already has GitHub SSH keys.
To test on the server side first
If you want to confirm before merging, on nuc-mini:
```sh sudo -u justin git ls-remote git@github.com:jedmund/line.git HEAD ```
Should print a commit SHA. If it does, this fix is sufficient.
After merging
```sh make deploy-infra-gateway ```
The git task will succeed and proceed to "Deploy infra-gateway stack" which will build and start the line container.