Add cachix push support to horizon-ci

Summary

• Re-adds cachix integration lost in the nixica 0.9.0 migration (!37 (merged))
• New options under horizon.ci.cachix:
  • enable — enable cachix push CI jobs
  • cache — cache name (default: "horizon")
• Uses gitlab-ci.modules.gitlab-ci.cachix from nixica's transitive gitlab-ci input
• Cachix options are partition-only (defined in the published module, not importAndPublish) to avoid duplicate declaration in root eval

Follow-up to: !37 (merged)

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