Get job ID from trusted file
The CUSTOM_ENV_*
variables can be overridden by users so we can't trust them. And we use CUSTOM_ENV_CI_JOB_ID
(CI_JOB_ID
to the user) to key sensitive VM metadata. Instead we should use the trusted JSON serialized JobResponse file: https://docs.gitlab.com/runner/executors/custom.html#job-response
Fixes https://gitlab.com/gitlab-org/security/ci-cd/custom-executor-drivers/autoscaler/-/issues/1