fix(maint): raise pacman cache warn threshold to 20GB

The 10GB threshold sat below the keep-3 steady state on both hosts (ratio 12.6GB, velox 10.8GB after pruning), so the warning persisted with nothing left to clean. paccache keeps three versions per package and never expires by age, which puts the honest ceiling near 17-19GB. At 20GB the metric stays quiet in steady state but still fires within about two months if the paccache timer dies.

I installed it on both hosts and the warning cleared on each.

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