Worker launcher should allow to set another default docker image

Currently, if no .spacelift/config.yml file is provided at the repository root, the launcher defaults to the Spacelift image public.ecr.aws/spacelift/runner-terraform:latest, and there’s no way to change that default for all stacks/runs.

However, to not force a .spacelift/config.yml file on each repository, and at the same time provide a way for customers to have their own default image for all the runs in a workerpool, the launcher should allow to customize that default image via environment variable (e.g. SPACELIFT_LAUNCHER_DEFAULT_RUNNER_IMAGE)