WIP: Resolve "Optimize `bst build` initialization time"

Description

Changes proposed in this merge request:

• Delegate calculation of artifact cache size to the artifact cache
• Make the artifact cache store its size, so we can know the artifact cache's approximate size without having to calculate it.
• Figure out a good way of making sure the stored artifact cache size doesn't drift too far out of sync.

This merge request, when approved, will close issue/bug:

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Closes #466 (closed)