Cache the license-eye build in the Check License Headers job

### Task Summary

The Check License Headers job (`check-header.yml`) uses apache/skywalking-eyes, whose composite action downloads Go module dependencies and builds `license-eye` from source on every run. That toolchain step costs roughly 40s of the ~70s job while the actual header scan of ~2,900 files takes ~30s. Add `actions/cache` for the Go module/build cache (or the built binary), keyed on the pinned skywalking-eyes commit. Warm runs then skip the rebuild and the job drops to roughly half its current time.

### Task Type

- [x] DevOps / Deployment / CI
- [x] Performance