Use jenkins-tools.git from its new location (gitlab-migration#97)

Maintaining HTTPS for now, as it's not a regression in this case.

We can probably use the role-jenkins-isotester user for this once the
SSH key of GitLab has been deployed in isotesters. I'm not sure if yet
another key would have to be added that user's config in GitLab.

See: https://salsa.debian.org/tails-team/gitlab-migration/-/issues/97