Plan Persistence

If you are using Github Actions, the output of `terraform plan` is stored in Workspace Artifacts. When you run apply, Digger will pick up the stored artifact.

If the PR code is updated, the now-outdated plan will be deleted from the plan store.

”Stale plans” was a massive issue in Atlantis (Issue #1122) and also flagged by many Digger users. If you run a plan and then update the PR and run apply without planning an old stored plan could have been picked up.

One alternative is to always re-plan before apply; but that kind of defies the purpose of storing plans, which are useful as artifacts. So we went with an alternative approach: in the event of a code change, an outdated plan is deleted from the storing bucket.