Builds openshell-gateway and openshell-sandbox natively per-arch on the
nv-gha-runners shared CPU pool with a GHA-backed sccache, and uploads
the resulting binaries as artifacts shaped for Dockerfile.images'
BINARY_SOURCE=prebuilt path (added in #945).
Dispatch manually 4-5 times after merge to collect cold + warm numbers
and compare against the Rust portion of docker-build.yml's ARC baseline.
PR 4c will wire these artifacts into the real pipeline once numbers land.
Refs OS-128.
Signed-off-by: Jonas Toelke <jtoelke@nvidia.com>