ci: stop the whole pipeline on the first failure and report why the restore image is missing [CI]

Every Linux step now runs under ci/guard-step.py. The first step that fails, is cancelled or times out marks the pipeline; running steps get SIGTERM (so traps release flock and Buildah state) then SIGKILL, steps that have not started exit at once, and any process a step leaves behind is killed. A pipeline deadline is the last resort. build-linux-agent.sh now prints the lookup error, store configuration and the restore images present when the prepared restore image cannot be found.

ci/test-ci-scripts.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# CI 脚本和安装器的静态检查与单元测试（verify 的第一步）。
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+sh -n packaging/openwrt/install.sh
+bash -n ci/publish-openwrt.sh
+python3 scripts/test-linux-agent-build.py
+python3 scripts/test-openwrt-updater.py
+python3 scripts/test-guard-step.py