add cross-compilation-aware pkg to allow build arm64

[AppleThomas]

No description provided.

[cursor]

PR Summary

Low Risk
Low risk: adds a single build dependency in the Docker build stage; impact is limited to image build behavior and cross-compilation support.

Overview
Adds pkg-config to cluster/docker/sv-dune.Dockerfile's build-stage apt packages so cross-compilation builds (e.g., arm64) can discover native libraries during compilation.

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