feat: lake: disabling the artifact cache also disables fetching

[tydeu]

This PR makes disabling the artifact cache (e.g., via LAKE_ARTIFACT_CACHE=false or enableArtifactCache = false) now stop Lake from fetching from the cache (whereas it previously only stopped writing to it).

There are now 3 possible configuration of the local artifact cache for a package:

• true: Artifacts will be fetched from the cache before building (if available) and built artifacts will be cached.
• false:: Lake will neither fetch artifacts from the cache or store them into it.
• default (no configuration set): Lake will fetch artifacts from the cache but not store them into it. A key motivation for this is to, by default, reuse artifacts downloaded into the cache from a remote service.

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-02-04 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-02-05 21:17:44)