Use CUSTOM_ENV_CI_JOB_IMAGE

The CUSTOM_ENV_CI_JOB_IMAGE variable contains the value of the "image"
key set for a given GitLab CI job.  Use that instead of the job name for
determinining the QCOW2 image to use.
1 job for michal/use-CUSTOM_ENV_CI_JOB_IMAGE in 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1012779
docker amd64
shellcheck

00:00:07