performance: Incorrect storage when run before setting system time

As reported by @fred1m-guest.

On Pioneer Edition and other hardware, there is no Real-Time Clock. When the system starts, the time and date on the system might be incorrect and set in the past. When PCP (performance co-pilot) starts with wrong time and later when the time is corrected, PCP is unable to store its data properly. This results in performance metrics not showing up in Cockpit.

It may be possible to add a dependency for the PCP daemon on time-set.target or time-sync.target.

Furthermore, when PCP data is corrupted, it is not able to recover without removing its data from /var/log.