docs(ci): stop promising the release lane a benefit its routed host cannot get

`integration_docker` is pinned to `ecs-qwen-hk4-host`, or `ubuntu-latest` when
the ECS pool is unavailable. On the Hong Kong host `deb.debian.org` already
answers at MB/s and the VPC mirror is unreachable, so `select_apt_mirror`
correctly keeps the built-in source; on a hosted runner the same is true. The
call-site comment asserted a mainland benefit this call site cannot obtain, and
the always-cold builder cache it sits next to is unchanged.

The comment now says what the call does: apply the same probe the E2E leg uses,
so a host whose default is throttled builds against a faster endpoint, while a
host whose default is already fast keeps it. The `Fixes #16271` claim is
narrowed to `Refs #16271` in the PR body — the always-cold half stays open.

Co-authored-by: Qwen-Coder <[email protected]>
Patrol-Run: qwen-pr-closeout/jmw420o4bfo

.github/scripts/run-release-docker-integration.sh

@@ -142,10 +142,12 @@ if [ "$image_inspect_status" -ne 0 ]; then
   # bounding that side too is a cross-lane change tracked separately.
   timeout 20m docker image prune --all --force --filter 'label=org.qwen-code.ci.sandbox=true' --filter 'until=24h' || echo "::warning::old CI sandbox image cleanup failed on ${RUNNER_NAME:-this runner}"
   check_docker_data_root_floor "${DISK_FLOOR_DOCKER_MIN_FREE_KB:-8388608}"
-  # This lane prunes the builder cache above, so the apt layers are always cold
-  # here — the same probe the E2E leg uses (#16214) selects the VPC intranet
-  # mirror on mainland hosts instead of rebuilding against the throttled
-  # default source (#16271).
+  # This lane prunes the builder cache above, so its apt layers stay cold, and
+  # its routed host (`ecs-qwen-hk4-host`, or `ubuntu-latest` when the ECS pool
+  # is unavailable) already reaches `deb.debian.org` at MB/s — there the probe
+  # keeps the built-in source, as designed. It applies the probe the E2E leg
+  # uses (#16214) so a host whose default is throttled builds against a faster
+  # endpoint; the always-cold builder cache is unchanged here (#16271).
   apply_apt_mirror
   # See e2e.yml: closing the lock descriptors in the child keeps a descendant
   # that outlives this job from holding the lock.