Protect variables in environment

As we introduced tests deploying runner on Equinix for this repo, I prefer to make sure sensitive variables are not propagated in jobs that don't need them.
That's why I'm proposing to use a dedicated Gitlab environment et associate jobs needing such credentials with it. Once merger, the variables would have to be scoped into the environment.

Tested here : https://gitlab.com/sylva-projects/sylva-elements/ci-tooling/runner-aas/-/pipelines/2183309754