The workflow was rebuilding dependencies unnecessarily despite timelord
restoring timestamps because TARGET_CPU and RUST_PROFILE weren't passed
to Docker, creating inconsistent cache keys. Now passes both arguments
for proper cache reuse.
Adds Haswell-optimised builds alongside baseline builds using -march=haswell
for PCLMUL instruction support. Recent build improvements reducing compile
times from 15-20 minutes to ~5 minutes make this additional CPU variant
feasible. Users can pull optimised images with -haswell suffix.
