Checkouting a tag fails in the `.migration-paths-upgrade-ce-to-ee` jobs

Job #106218414 failed for ebd33cde:

It looks like the cause is that https://gitlab.com/gitlab-org/gitlab-ee/blob/master/.gitlab-ci.yml#L341 is filling the variable CE_HEAD with the value origin/v11.4.0-rc2. That syntax is proper for a branch, not a tag. Looking at the code of scripts/ee_specific_check/ee_specific_check.rb, we find this: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/scripts/ee_specific_check/ee_specific_check.rb#L242 So it seems that ee_specific_check is handling tags as if they were branches, and thus outputs them with the wrong syntax. @ayufan has proposed that we change https://gitlab.com/gitlab-org/gitlab-ee/blob/master/.gitlab-ci.yml#L342 to git checkout -f FETCH_HEAD. This will work if the above always leaves FETCH_HEAD properly set, which I don't know.