TF_PLAN_CACHE and TF_PLAN_JSON are currently hardcode in our template

The problem

TF_PLAN_CACHE and TF_PLAN_JSON are currently hardcoded in our template

This means that when we set these variables using the terraform-image, it won't be respected in the template.

See the following discussion from gitlab-org/terraform-images!162 (merged):

• @timofurrer started a discussion: (+5 comments)

  @Alexand @tigerwnz may I ask you to give this a thorough review? Regarding functional correctness and backwards-compatibility?

One possible solution, but a breaking change, is to change the template to point to the result of the variables directly: artifacts.path: TF_PLAN_CACHE.