index: do not include GitLab Pages mirror if it can't be deployed

GitLab Pages sites are limited to 1GB on gitlab.com, so the CI/CD job will
fail if the repo is bigger than that.  It should not be included as a
mirror in that case.

https://docs.gitlab.com/ee/user/gitlab_com/#gitlab-pages
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f