feat: cache rocks in build-artifacts workflow

[javierdelapuente]

Summary

Caches built rocks between CI runs to avoid redundant rockcraft pack invocations. Modelled directly on operator-workflows' build.ts caching strategy.

Closes #13

What's cached

Only the partial artifacts.build.yaml manifest (contains the GHCR image: ref), not the .rock file itself — the GHCR image persists across runs with its original tag.

Cache key structure

rock-<name>-<arch>-<hash-of-source-dir>-<YYYY-WNN>

• hash: SHA-256 over all non-.rock files in the pack-dir (or rockcraft.yaml parent dir), matching operator-workflows' hashFiles() approach.
• YYYY-WNN (ISO week): explicit weekly rotation — GitHub's 7-day LRU also applies but this is more predictable.
• No restore-keys fallback — a partial hit would restore a manifest with a stale GHCR image ref built from different source files.

Behaviour

Scenario	Result
Cache hit (registry mode)	LXD, rockcraft, GHCR login, and rockcraft pack are all skipped; restored manifest is uploaded by the existing unconditional step
Cache miss (registry mode)	Build proceeds as before; manifest is saved to cache afterwards
Artifact mode (fork PRs)	No caching — GH artifacts are run-scoped; the .rock file is too large to cache efficiently

Changes

.github/workflows/build-artifacts.yml

• New Compute rock cache metadata step: computes source-dir hash + ISO week, emits cache key.
• New Restore rock cache step: actions/cache/restore@v4.
• LXD install, rockcraft install, GHCR login, and main build steps conditionalized on cache miss.
• New Save rock cache step: actions/cache/save@v4 on cache miss only.

src/opcli/core/artifacts.py

• artifacts_matrix() now includes rockcraft-yaml and pack-dir fields in rock matrix entries (needed by the workflow to locate the source directory for hash computation).

tests/unit/test_artifacts.py

• Updated existing matrix test + 2 new tests for the new fields.