Fix registry key for LeapMicro

Related to sylva-projects/sylva-core#3637 (closed)

Workload cluster deployments fail with image dccb976dc06001565cec520c967f07a78dfca99175ad8b29eb7c69dcbe46a0f2/leapmicro-6-0-plain-rke2-1-32-dccb97 is currently not served by mgmt cluster os-image-server because the registry selector does not include kiwi-image-builder.

Failed jobs: https://gitlab.com/sylva-projects/sylva-core/-/jobs/13210976974 and https://gitlab.com/sylva-projects/sylva-core/-/jobs/13210976965

Tested in sylva-projects/sylva-core!7014 (closed)

both ☁️capm3 🚀rke2 🐧leapmicro and ☁️capm3 🚀rke2 🎬wkld-k8s-upgrade-n-2-n-1 🛠️ha 🐧leapmicro are green.