ci: build linux/arm64 images on native runners

[ahhda]

Summary

• Publishes ghcr.io/gnosis/solvers as a multi-platform image (linux/amd64 + linux/arm64), same tags as before (main, v*).
• Unlike cowprotocol/automated-trade#53 (QEMU), each arch builds on a native runner (ubuntu-24.04 / ubuntu-24.04-arm): the Rust build plus the heaptrack compile in Dockerfile.binary would be far too slow under emulation. Same pattern as cowprotocol/services deploy.yaml.
• build pushes each arch by digest (untagged); publish merges the digests with docker buildx imagetools create and applies the tags, so tags only move if both arches succeed. Autodeploy runs after the manifest is published.
• Dockerfile.binary is unchanged; existing action versions untouched, new actions pinned by SHA.

Testing

deploy.yaml doesn't run on PRs; I'll run it via workflow_dispatch on this branch, which pushes a throwaway :anuj-arm64-images tag and skips autodeploy (main-only).

https://claude.ai/code/session_01Ns6R5rJBskJfYv5F43Mdjf

[ahhda]

Test run via workflow_dispatch on this branch passed: https://github.com/gnosis/solvers/actions/runs/36036662993 — arm64 and amd64 each built natively in ~9–10 min, and ghcr.io/gnosis/solvers:anuj-arm64-images is a multi-platform OCI index.