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
determining the QCOW2 image to use, so that defining GitLab CI jobs
using the libvirt/QEMU custom executor becomes a bit more similar in
spirit to defining jobs using the Docker executor.
1 job for michal/use-CUSTOM_ENV_CI_JOB_IMAGE in 7 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #1015514
docker amd64
shellcheck

00:00:07