How to enable "Project-level Secure Files" Feature

[sugarwyc]

[kkimurak]

In current release (v15.0.1) you cannot because of lacking configuration.
I'm working on #2570 and you can see those settings are going to be added. So after it merged, I believe we will be able to enable it from environment variable (not defined yet, but I think it will be something like GITLAB_CI_SECURE_FILES_ENABLED and so on)