Test restrict_user_defined_variables=True on containers

GitLab 13.8 introduced a new restrict_user_defined_variables project setting to restrict who can set environment variables to Maintainers only. There currently is no UI for this setting, it must be set with the project settings API.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51682/ https://docs.gitlab.com/ee/ci/variables/#restrict-who-can-override-variables

• Get a developer account
• Test that CHT projects correctly have this variable set
• Test that the developer account cannot override variables