Provide some kind of templating/interpolation/variable expansion in the runner script

[emanueleaina]

While job definitions are considered to be handlebars templates that expand job variables, at the moment there is no way to use any kind of CI variables in the lava-gitlab-runner script itself.

For instance, I have job-amd64.yaml and job-arm64.yaml and I'd like to submit them using parallel:matrix and something like job-$ARCH.yaml or job-{{job.ARCH}}.yaml, but it is not currently possible