fix(ci): lowercase ghcr.io image name in per-arch digest push

[sirantd]

Summary

• Follow-up to fix(core): give nested eBPF build.rs cargo invocation its own target dir #52/fix(core): strip leaked build-script env from nested eBPF cargo call #53: the eBPF build itself now completes cleanly (~6 min, no deadlock, no symbol error) but the release workflow's per-arch build job failed at push time — invalid reference format: repository name (Perun-Engineering/ebpf-oom-watcher) must be lowercase.
• docker/metadata-action auto-lowercases the tags it generates, but the per-arch job's build-push-action outputs: field referenced raw ghcr.io/${{ github.repository }} directly for the digest-only push, bypassing that normalization.
• Fix: compute a lowercased IMAGE env once per job and use it consistently for both the metadata-action images: input and the digest-push outputs: field.

Test plan

• Merge and confirm the Release workflow completes end to end (build → merge-and-push → helm → create-release)