ci: also build and test on arm64

[chishui]

Description

The Linux CI job has only ever built on x64. This adds an architecture matrix so it also builds and tests on arm64.

ci-image-version-linux is a multi-arch manifest, so the tag get-ci-image-tag.yml already returns resolves to the arm64 image on an arm64 runner. Nothing about the container changes — no new workflow input, no second image-tag job. get-ci-image-tag.yml has no architecture input at all (its knobs are product, platform, type, build_ref, version), and none is needed.

The single almalinux8 container platform is unchanged, matching what k-NN does and what the OpenSearch distribution is built on.

Why this is real coverage, not a second run of the same code

The existing SIMD detection step selects sve on the arm64 runner, so the SVE kernels get compiled and their tests executed for the first time in CI. Confirmed on this branch:

Leg	SIMD level selected	Result
Linux (x64)	avx2	608/608 passing, generic + SIMD builds
Linux (arm64)	sve	608/608 passing, generic + SIMD builds

This complements #40, which added the NEON and SVE kernel equivalence tests — those SVE assertions now actually run on SVE hardware.

Scope

An earlier revision of this PR also built on the ubuntu2404 ci-runner platform. That is dropped: k-NN and the distribution build both use one Linux container platform, and multi-distro coverage for a library is not what the other ci-runner platforms exist for.

Worth recording what that experiment turned up, since it is a live hazard for anyone adding a platform here:

• The published ubuntu2404 ci-runner image has no cmake, even though the Dockerfile on opensearch-build main pip-installs it. get-ci-images.sh resolves to a floating tag — its grep -Eo '.*v[0-9]' truncates the minor version, so the workflow gets ci-runner-<platform>-opensearch-build-v1 and the published image can lag the Dockerfile. Both ubuntu2404 legs failed with cmake: command not found.
• The remaining opensearch/build platforms cannot compile this package at all: it requires C++20 (CMAKE_CXX_STANDARD 20) and uses <ranges> at 23 sites and <span> at 15, while rockylinux8 (gcc 8.5), centos7 (devtoolset-9) and ubuntu2004 (gcc 9.4) all predate those headers in libstdc++.

Check List

• New functionality includes testing — this change is test coverage; both legs verified green on this branch.
• New functionality has been documented.
• Commits are signed per the DCO using --signoff.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.