chore(ci3): add optional local cache for bootstrap artifacts

[spalladino]

Summary

• Adds an optional local filesystem cache layer for build artifacts that sits in front of the S3 remote cache, controlled via the CACHE_LOCAL_DIR env var
• On download: checks local cache first (instant), on miss downloads from S3 and saves to local cache for next time
• On upload: saves the artifact to the local cache alongside the S3 upload
• When CACHE_LOCAL_DIR is unset, behavior is identical to before (zero impact on CI or other devs)
• Gracefully falls through to S3 if the local cache directory cannot be created (e.g. permission issues)

This reduces full bootstrap time from 165s to 77s by avoiding redundant S3 downloads for artifacts that haven't changed.

Usage

export CACHE_LOCAL_DIR="$HOME/.aztec-cache"
./bootstrap.sh

Test plan

• Added ci3/cache_local.test.sh with 8 tests (12 assertions) covering: local cache hit/miss, upload save, roundtrip, disabled-cache bypass, inaccessible directory fallthrough
• Verified no rm -rf on any path derived from CACHE_LOCAL_DIR
• Run bash ci3/cache_local.test.sh to execute tests

🤖 Generated with Claude Code

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/1843848965a8dceb�1843848965a8dceb8;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_epochs/epochs_mbps.parallel.test.ts "builds multiple blocks per slot with transactions anchored to proposed blocks" (218s) (code: 1) group:e2e-p2p-epoch-flakes

[ludamad]

not a fan of this redundancy. extract a function?

[ludamad]

mostly lgtm, just the one comment. we should be able to extract a function that puts the tar into stdout that can be used in both cases

[spalladino]

@ludamad done

[ludamad]

Note that we don't really cache upload locally so that change isn't super necessary but I'm ok with it