…rigontech#21741)

## Why

`docker/setup-buildx-action` boots BuildKit by pulling `moby/buildkit`
from Docker Hub on every run — the last uncached Docker Hub dependency
in the kurtosis jobs. In merge-queue run
[27280175556](https://github.com/erigontech/erigon/actions/runs/27280175556/job/80572198019)
that pull timed out (`Get "https://registry-1.docker.io/v2/": context
deadline exceeded`), failing the CI Gate for erigontech#21723 — ironically the PR
closing the equivalent gap for the kurtosis engine-bootstrap images. The
same Docker Hub connectivity window took out three other CI Gate runs
that morning at the `kurtosis engine start` step.

erigontech#21735 added a retry around buildx setup, but a retry doesn't survive an
outage longer than its window. This removes the hard dependency the same
way as the rest of the image-caching series (erigontech#21695, erigontech#21703, erigontech#21723).

## What

- Pin the BuildKit image as `BUILDKIT_IMAGE: moby/buildkit:v0.30.0`
(what the moving `buildx-stable-1` tag currently resolves to) and pass
it to `setup-buildx-action` via `driver-opts: image=...`, in both
`test-kurtosis-assertoor.yml` (`build-erigon-image`) and
`test-kurtosis-gloas.yml` (`gloas_test`). Bump alongside the other
pinned images.
- Cache it under a single shared key (`docker-buildkit-<image>`) with
the established docker save/load + actions/cache pattern, and `docker
load` it before buildx setup. buildx's docker-container driver falls
back to a locally present image when its pull fails ("pulling failed,
using local image"), so with a warm cache the builder boots even while
Docker Hub is fully unreachable. Pinning via driver-opts is what makes
the fallback engage — the local image name must match what buildx wants
to boot.
- The cache-fill pull in the test jobs is best-effort
(`continue-on-error`, save gated on pull success): buildx pulls the
image itself either way, so a failed seed must not fail an
otherwise-good run, and a failed pull never poisons the cache key with
an empty archive.
- Warm jobs (`warm-third-party-images` in both files) pull strictly and
save the same key — producing the cache is their purpose. Both
cache-warming workflows already path-filter on the edited files, so the
cache is created on main right after this merges and refreshed daily
against LRU eviction.

In gloas, buildx setup previously ran before any caching; the buildkit
cache steps are inserted ahead of it.

Not covered (non-gating, can follow up if wanted):
`ci-cd-main-branch-docker-images.yml` and `release.yml` also use
`setup-buildx-action` but don't block PRs or the merge queue.

actionlint is clean (the two SC2086 infos it reports pre-exist on main
in the kurtosis CLI install step).