…phemeral merge-queue ref (erigontech#21447)

## Problem

In the merge queue, hive / hive-eest jobs intermittently fail with **all
clients failed to build**:

```
Cloning: erigontech/erigon - gh-readonly-queue/main/pr-XXXXX-<sha>
fatal: Remote branch gh-readonly-queue/main/pr-XXXXX-<sha> not found in upstream origin
→ image build failed (128) → "too few tests" → ci-gate failure
```

`test-hive.yml` / `test-hive-eest.yml` build the hive erigon client via
`clients/erigon/Dockerfile.git`, which runs `git clone --depth 1
--branch $tag https://github.com/$github`. For `merge_group` events
`GITHUB_HEAD_REF` is empty, so `$tag` falls back to
`${GITHUB_REF#refs/heads/}` = the **ephemeral** `gh-readonly-queue/...`
ref. When the queue re-forms the group or a runner is slow to start,
GitHub deletes that ref before the in-container clone runs → the clone
fails. This evicts PRs from the merge queue (e.g. erigontech#21421), independent
of the failing PR's code.

`actions/checkout` doesn't hit this — GitHub reliably provides the
`merge_group` commit to the run; only the *independent* `git clone
--branch <ephemeral-ref>` inside hive's builder races the ref's
deletion.

## Fix

Build the erigon image from the **checked-out commit** and wrap it with
hive's default *prebuilt-image* `Dockerfile` (`FROM $baseimage:$tag`),
in both hive workflows:

- check out the full erigon source into a fresh **`erigon-full`** path.
The old sparse `erigon-src` path leaves a stale
`.git/info/sparse-checkout` on the **reused** self-hosted hive runners;
a full checkout into that same path doesn't clear it, so the tree stays
sparse, `cmd/erigon` is missing, and the build fails (`cd ./cmd/erigon:
No such file or directory`). A fresh path no sparse run has touched
avoids that.
- plain **`docker build -t hive/erigon:cilocal`** on the host daemon
(persistent layer cache on the reused runners) — *not* a shared
`type=gha` cache, so no concurrent-writer 504s and no cross-workflow
scope contention (the failure mode `test-kurtosis-assertoor.yml`
centralizes its build to avoid).
- point hive's default client Dockerfile at that local image
(`baseimage=hive/erigon`, `tag=cilocal`) instead of `mv`-ing in
`Dockerfile.git`.
- drop the now-unused `SOURCE_REPO` / `branch_name` / clone-arg /
builder-Go-version plumbing.

`-docker.pull` defaults to false (CI doesn't pass it), so `FROM
hive/erigon:cilocal` resolves from the local daemon, never force-pulled.
erigon's image is `debian:13-slim` with `erigon` on PATH, so hive's
default Dockerfile (`apt-get`, `erigon --version`) layers on top cleanly
— on both `ethereum/hive` and the `erigontech/hive` fork used by
hive-eest.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: info@weblogix.biz <admin@10gbps.weblogix.it>