Cache the Circom binary in the circuit-regression CI job

[collinsadi]

.github/workflows/ci.yml curls the Circom binary on every run of the circuit-regression job, adding avoidable latency to each build. We should cache the pinned Circom binary (keyed on version) with actions/cache, or skip the circuit job when no *.circom or circuit files changed.

Acceptance criteria

• The Circom binary is restored from cache on unchanged-version runs.
• The job wall-time visibly drops on a warm cache.