On push builds, CircleCI provides `CIRCLE_COMPARE_URL`, which we use to
extract the commit range for a given build. When a workflow is rerun
(e.g. to recover from a flaked job), `CIRCLE_COMPARE_URL` is not
defined, causing some jobs to fail.
This commit fixes it by retrieving the compare URL from the original
workflow. It uses a slow process involving a (potentially large) number
of requests to CircleCI API.
It depends on the (undocumented) fact, that the `workspace_id` is the
same on all rerun workflows and the same as the original `workflow_id`.
PR Close#27775
George Kalpakas