LRU cache for artefacts

**Is your feature request related to a problem? Please describe.**

Build's filesystem cache has unbounded growth.

**Describe the solution you'd like**

Implement a configurable size-limited LRU cache, with ejections once the size threshold is reached.

This will live in `PlaceOS::Build::FileSystem`

**Additional context**

Could be made generic and used as a component for #4 