Report wall-clock elapsed time rather than cumulative job time

From https://gitlab.slack.com/archives/development/p1461874807002908

We currently report something like:

16 builds for commit a3d6b610 in 348 minutes 44 seconds

But the total time from start to finish was only 20 minutes or so. The user doesn't care (as much) about the amount of compute used to process their tests, they do care very much about how long they had to wait to see the pass/fail results. Let's report that time instead.