BM-2468: Speed up Docker builds

[capossele]

All service Dockerfiles (slasher, distributor, order-stream, order-generator, broker, market-indexer, market-indexer-backfill, rewards-indexer) now have:

• sccache with S3 backend -- shares compiled artifacts across builds via the risc0-ci-cache S3 bucket, eliminating redundant compilation across pipeline runs
• --package flag on cargo chef cook -- scopes dependency compilation to only the crates each service actually needs, instead of cooking the entire workspace

Infra stacks updated to pass CI_CACHE_SECRET as a Docker build secret: slasher, distributor, and order-generator. (Order-stream and indexer were already wired up.)

TODO:

• Set CI_CACHE_SECRET in Pulumi config for all stacks that build Docker images. The value should be the absolute path to an AWS credentials file with read/write access to the risc0-ci-cache S3 bucket. Without this, sccache falls back to local-only caching which provides no cross-build benefit in CI.

[linear]

BM-2468 Speed up Docker builds