Merge the per-arch build-all and install-deps files into one each:
build-all-versions-{amd64,arm64}.yaml -> build-all-versions.yaml
build-all-versions-install-deps-{amd64,arch}.yaml -> build-all-versions-install-deps.yaml
Arch becomes a matrix axis carrying the per-arch agent (artemis/gaia),
BACKEND (kubernetes/docker) and R_PKG_CACHE_DIR. Routing is unchanged: the
existing `task == "build-all-${ARCH}"` selector means
`--var task=build-all-amd64` still runs only the amd64 rows. OS / OS_VERSION /
R_VERSION / K8S_INSTANCE_TYPE are still passed via --var. These pipelines are
manual-only (no cron).
