We host in a private (self-hosed) git-Lab, which can only be cloned through the SSH, and we would like to import packages by compiling it from a public git-Hub (or by installing it using wheels with find-links).

Try removing force_git_ssh_protocol: true
If you do not provide user/pass , and assuming the repo link on the task is internal, hence SSH, it should leave links as they are (ssh for ssh http for http), this should solve the issue (I hope)