ci: g1-migrator, switch to image from the registry

Remove clone, the sources are now embedded Remove now useless DEBIAN_FRONTEND: noninteractive, since there are no more apt commands

Second commit is optional. I think triggering py-g1-migrator pipeline on its repository is good enough, since the sources shouldn’t change this frequently. We can try multi-project trigger pipeline, to make things fancy.

Depends on tools/py-g1-migrator!8 (merged)