Add shared block_log linking for HAF cache extraction

When extracting HAF caches, blockchain directory is excluded to save
~1.7GB per cache. This adds automatic linking to the shared block_log
after extraction.

The _link_shared_block_log function:
- Checks if blockchain already exists in extracted data (skip if so)
- Tries local /blockchain/block_log_5m first (each builder has this)
- Falls back to NFS /nfs/ci-cache/hive/block_log_5m
- Creates symlinks in datadir/blockchain

This consolidates the block_log fallback logic in cache-manager instead
of having it duplicated in HAF's copy_datadir.sh script.