Add a `CI_RUNNER_VERSION` predefined variable
Description
GitLab CI provides a CI_SERVER_VERSION predefined variable which is great for determining what version of GitLab was used to schedule the job, but how can a job get the version of GitLab Runner that is executing the job?
This is for medical software where I need to check that the version of the runner matches what we validated.
Proposal
Add a CI_RUNNER_VERSION predefined variable.