GitLab

Use CUSTOM_ENV_CI_JOB_IMAGE

See merge request !5

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.

Add step_script stage

See merge request !3

GitLab Runner 13.1.0 seems to add a "step_script" stage. Without this
stage recognized libvirt jobs fail with:

    Executing "step_script" stage of the job script
    Unsupported run stage 'step_script'

The "step_script" stage is not documented upstream.

Add QEMU-based libvirt executor

See merge request !2

Run ShellCheck in GitLab CI

See merge request !1