Add RISC-V build for the kube-cross image

[chazapis]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds a RISC-V build to the kube-cross container image produced; no code changes.

We need it, as K3s supports building for riscv64 (relevant discussions are here and here), but the pause image is incompatible with the architecture. The pause image is built using the kube-cross container (relevant PR here), which now lacks a cross-compiler for riscv64. This is a necessary step towards eventual RISC-V support in Kubernetes.

Which issue(s) this PR fixes:

There is no open RISC-V issue in release, but there is an open discussion on adding support for the RISC-V architecture in Kubernetes.

Special notes for your reviewer:

A cross-compiler for riscv64 is available in Debian trixie onwards, so I have updated the base image to the latest Debian stable (all previous releases have passed their EOL and are currently in LTS anyway).

Does this PR introduce a user-facing change?

NONE

[kubernetes-prow]

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: chazapis
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[kubernetes-prow]

Hi @chazapis. Thanks for your PR.

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[dims]

please see:
kubernetes/kubernetes#132836 (comment)

/hold