Remove GHA cache from test job to fix prod build times

[damianmoore]

Summary

• Remove cache-from/cache-to from the test job's Docker build step
• The test + prod caches combined exceed GitHub's 10GB cache limit, causing prod cache entries to be evicted (LRU)
• This forces the multi-platform prod build to rebuild from scratch (~39 min instead of ~1 min when cached)
• The test cache provides minimal benefit — the image is ephemeral and code layers change every push

Test plan

• Verify test job still builds and runs successfully (just without cache)
• Verify prod build-and-push hits cache on subsequent runs (should drop from ~39 min to ~1 min)

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