perf(ci): pin staged restore mtimes so the restore layer caches, drop build intermediates from the layer [CI]

ci/stage-node-image.py

@@ -4,11 +4,13 @@
 from __future__ import annotations
 
 import argparse
+import os
 import shutil
 import subprocess
 from pathlib import Path
 
 ROOT = Path(__file__).resolve().parents[1]
+FIXED_MTIME = 946684800  # 2000-01-01T00:00:00Z
 BACKEND = ROOT / "backend"
 STATIC_MSQUIC = ROOT / "packaging/native/msquic"
 IGNORED_PARTS = {".git", ".release", "artifacts", "bin", "obj", "TestResults", "tests"}
@@ -86,6 +88,14 @@ def stage(component: str, runtime_identifier: str, destination: Path) -> None:
     # 更新清单和请求放在这里；空的命名卷第一次挂载时会带上这个目录
     (config / "update").mkdir()
     (config / "update" / ".keep").write_text("", encoding="utf-8")
+    normalize_mtimes(destination / "restore")
+
+
+def normalize_mtimes(directory: Path) -> None:
+    """The Buildah layer cache keys COPY layers on file metadata as well as content. Files copied from a fresh checkout carry a new
+    mtime every run, so the restore layer (and the ~70 s of NuGet downloads under it) never hit the cache. Pin the mtimes."""
+    for path in [directory, *directory.rglob("*")]:
+        os.utime(path, (FIXED_MTIME, FIXED_MTIME))
 
 
 def main() -> None: