Elapsed time cannot be calculated from workflow updated_at

[fabjan]

The updated_at timestamp cannot be used to estimate how long a workflow took to run, it changes when GitHub e.g. archives a run. See https://github.com/fabjan/buildogram/actions/runs/4038365177 for example.

To fix this we probably need to get the jobs inside each workflow run and compare their earliest started_at and latest completed_at timestamps.