e2e-sandbox: the mc download URL returns 410, so the image cannot be rebuilt #4487

The e2e-sandbox image cannot be rebuilt on main. Its Dockerfile downloads the MinIO client from https://dl.min.io/client/mc/release/${TARGETOS}-${TARGETARCH}/mc, and that URL returns HTTP 410 for both linux-amd64 and linux-arm64 (checked 2026-09-26 with curl -I). With curl -fsSL the RUN step fails, so any PR that touches packages/core/testing goes red at the image build.

The fix is already written in #4447: it takes the last mc release from GitHub and checks a pinned sha256 per architecture. This issue is here so the breakage has a record until that PR lands, and so the fix does not get lost if the rollup is split.