Change `buildjob` to accomodate external timing runners

Continuing where we left off at !1 (merged). I was using the fp-timing branch, but then CI_RUNNER_TAGS changed format, which broke submitting timing info on that branch.

This MR contains the changes from !1 (merged), and additionally changes the logic that detects whether the current run is a timing run.
The name of the runner is no longer used (or required to be coop-timing), but the runner must have the fp-timing tag (may have more tags).

This currently merges into master. Let me now if opam2 would be more suitable.