ci: keep build cache cleanup best-effort in the restore step [CI]

The restore step failed after restoring every dependency because the final cache cleanup (buildah unshare reporting memfd_create) was not guarded. Cleanup failures now only warn, and a test requires every cleanup call to be guarded.

ci/restore-linux-dependencies.sh

@@ -149,8 +149,9 @@ PY
   done
   flock -u 9
   agent_lock_held=false
+  # 清理只是尽力而为：依赖已经还原好了，清理失败（比如 buildah unshare 在这台主机上报 memfd_create 错误）不能让这一步失败
   if flock -n -x 9; then
-    bash "$root/ci/cleanup-build-cache.sh"
+    bash "$root/ci/cleanup-build-cache.sh" || echo 'warning: build cache cleanup failed' >&2
     flock -u 9
   fi
 fi