Placement is handled by the agent label (artemis/gaia) and only the docker
backend is used, so the kubernetes backend_options blocks, the arch/instance
nodeSelector, and the K8S_INSTANCE_TYPE variable are dead. Remove them and the
now-redundant BACKEND matrix column.
The consolidated build-all routed on `task ==`, which crow #1165 filters out
of the manual-run form, leaving "no configurable variables". Declare
target_arch (routes the matrix) plus OS / OS_VERSION / R_VERSION /
K8S_INSTANCE_TYPE as variables, and route on `target_arch == "${ARCH}"` so the
form is populated and arch selection works from the UI. CLI callers now pass
`--var target_arch=amd64` instead of `--var task=build-all-amd64`.
Merge the per-arch build-all and install-deps files into one each:
build-all-versions-{amd64,arm64}.yaml -> build-all-versions.yaml
build-all-versions-install-deps-{amd64,arch}.yaml -> build-all-versions-install-deps.yaml
Arch becomes a matrix axis carrying the per-arch agent (artemis/gaia),
BACKEND (kubernetes/docker) and R_PKG_CACHE_DIR. Routing is unchanged: the
existing `task == "build-all-${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).