Allow owners to fetch source code in CI builds

Due to different way of handling owners of a project, they were not allowed to fetch CI sources for project.

This adds a separate code path for handling owners, that are not admins.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23437

See merge request !6943
Signed-off-by: Rémy Coutable <remy@rymai.me>