grafana: Allow to filter on parallelism

Followup of #22:

On the grafana dashboard, the search box "build_command filter" was mainly added to filter for "-j1" or "-j4".

Instead of, or in addition to this field, we'd now need a filter for the parallelism count.

I don't see a reason to keep the build_command filter, so I'd just probably relabel this, and make it work on the new field.