Describe the enhancement

In addition to supporting the --once option (#510), it would be useful if a runner could be limited to a specific run or job, exiting once this job is completed (or cancelled or not otherwise queued).

A typical use case would be for autoscaling: either via a webhook or polling the workflow runs API, i would start up the necessary number of workers for the jobs/runs.

Code Snippet

./run.sh --job=<job_id> # run job matching job_id, or exit if complete/cancelled

Additional information
Similar to the Buildkite --acquire-job option.