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.
This variable was previously not exposed in the Custom executor's
environment - this feature was added in GitLab 12.6, i.e. after the
scripts in this repository were initially written. Shout-out to
@godfryd for finding this.
@godfryd: could you please take a look whether this branch works for your purposes?