The previous attempt where we set a environment variable for the
artifacts key resulted in a chicken/egg problem: we needed the environment
variable to be able to restore the cached artifacts, but we needed
the cached artifacts in order to be able to read the variables...
This change now properly uses run_id: in `deb-packaging.yml` we can
directly use `run_id` from the context. In `api-verification.yml` we get
it from the payload context.
Cf https://docs.github.com/en/webhooks/webhook-events-and-payloads#workflow_run