Clone repo in `build-assets-image`

What does this MR do?

Clones repository in build-assets-image so that the job can run in dev.gitlab.org where the files are not publicly available.

dev.gitlab.org failure reported in #215003 (comment 333151705)

Test job in dev.gitlab.org successful: https://dev.gitlab.org/gitlab/gitlab-ee/-/jobs/7592939

Closes #215003 (closed)