[K8s] Auto-detect cluster architecture for runtime build (closes #1462)

[mskuratowski]

Closes #1462.

KubernetesBackend.build_runtime no longer hardcodes --platform=linux/amd64. The arch is resolved per build:

1. runtime_arch from k8s config wins (allowed: amd64, arm64).
2. Otherwise, query nodes via the kube API; if all share an arch, use it.
3. Fall back to amd64 if the cluster is mixed-arch or the API query fails (logged as a warning).

I tested the solution on kind and rancher-desktop.

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[JosepSampe]

Great, thanks @mskuratowski !