Install Checkov in the Atlantis image

What

Install Checkov in the Atlantis image.

Needed for gitlab-com/gl-infra/k8s-workloads/gitlab-helmfiles!3474 (merged)

Why

Part of:

• https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16987
• https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24613