docker: download the zig compiler for the right target

[gballet]

The arm build was reactivated, and the error from risc0 circumvented. But it was downloading the compiler in a way that

[Copilot]

Pull Request Overview

This PR modifies the Docker build process to properly support multi-architecture builds by downloading the correct Zig compiler for each target architecture and copying the appropriate runtime libraries. The change addresses an issue where the ARM build was downloading the wrong compiler architecture.

• Replaces hardcoded x86_64 Zig compiler download with architecture-aware logic
• Adds runtime library selection based on target architecture
• Introduces an intermediate runtime-prep stage to handle architecture-specific library copying

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[gemini-code-assist]

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