fix(pm): keep npm's self-install cache out of the download entry

npm caches the tarball it installs. The cache was written next to the
archive, inside the store's fetch-<sha> entry. Removing that entry after
publication then had to delete a tree, and on Windows it failed while
Defender still held the fresh tarball copy:

  [WinError 145] The directory is not empty: ...\fetch-<sha>\.npm-cache\...

Give npm a throwaway temp cache whose cleanup cannot fail the install,
so the download entry holds only the archive.

pm/packages.py

@@ -8,6 +8,7 @@ import os
 import shutil
 import subprocess
 import sys
+import tempfile
 from pathlib import Path
 from typing import Optional
 
@@ -567,20 +568,24 @@ class Npm(BinaryPackage):
         if not bundled_cli.is_file():
             raise InstallError(self.name, "node's entry is missing its bundled npm-cli.js")
 
-        env = npm_env(archive.parent / ".npm-cache")
-
         staged.mkdir(parents=True, exist_ok=True)
-        proc = subprocess.run(
-            [
-                str(node_bin), str(bundled_cli), "install", "--global",
-                "--prefix", str(staged), "--offline", "--ignore-scripts",
-                "--no-audit", "--no-fund", str(archive),
-            ],
-            capture_output=True,
-            text=True,
-            timeout=900,
-            env=env,
-        )
+        # npm caches the tarball it installs. The archive's directory is the
+        # store's download entry, which must hold only the archive: a cache
+        # there turns its removal into a tree delete that fails on Windows
+        # while Defender still holds the fresh copy (WinError 145). Cleanup
+        # of this throwaway cache must never fail the install.
+        with tempfile.TemporaryDirectory(prefix="hermes-npm-cache-", ignore_cleanup_errors=True) as cache:
+            proc = subprocess.run(
+                [
+                    str(node_bin), str(bundled_cli), "install", "--global",
+                    "--prefix", str(staged), "--offline", "--ignore-scripts",
+                    "--no-audit", "--no-fund", str(archive),
+                ],
+                capture_output=True,
+                text=True,
+                timeout=900,
+                env=npm_env(Path(cache)),
+            )
         if proc.returncode != 0:
             raise InstallError(
                 self.name, f"self-install exited {proc.returncode}: {proc.stderr[-400:]}"

tests/pm/test_npm_self_install.py

@@ -0,0 +1,59 @@
+"""npm's self-install keeps its cache out of the store's download entry.
+
+A cache beside the archive turned the entry's removal into a tree delete
+that failed on Windows while Defender held the fresh tarball copy
+(``[WinError 145] The directory is not empty: ...fetch-<sha>\\.npm-cache``).
+"""
+
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+import pm.install
+import pm.packages
+from pm.registry import get_package
+from pm.store import Store, current_target
+
+
+def _fake_node(store: Store, target: str) -> None:
+    node_bin = get_package("node").binary(store.entry("node-1"), target)
+    node_bin.parent.mkdir(parents=True, exist_ok=True)
+    node_bin.write_bytes(b"node")
+    cli_root = node_bin.parent if target.startswith("win32") else node_bin.parent.parent / "lib"
+    cli = cli_root / "node_modules" / "npm" / "bin" / "npm-cli.js"
+    cli.parent.mkdir(parents=True)
+    cli.write_text("", encoding="utf-8")
+
+
+def test_npm_self_install_leaves_download_entry_holding_only_the_archive(tmp_path, monkeypatch):
+    target = current_target()
+    store = Store(tmp_path / "store")
+    _fake_node(store, target)
+    monkeypatch.setattr(pm.install, "_lockfile", lambda: None)
+    monkeypatch.setattr(pm.install, "_installed_location",
+                        lambda package, lockfile, target: ({"node": {"entry": "node-1"}}, store))
+
+    caches: list[Path] = []
+
+    def fake_npm(cmd, **kwargs):
+        # Stand in for npm: cacache writes the installed tarball into the cache.
+        cache = Path(kwargs["env"]["npm_config_cache"])
+        content = cache / "_cacache" / "content-v2" / "sha512" / "b8" / "85"
+        content.mkdir(parents=True)
+        (content / "blob").write_bytes(b"tarball")
+        caches.append(cache)
+        return subprocess.CompletedProcess(cmd, 0, "", "")
+
+    monkeypatch.setattr(pm.packages.subprocess, "run", fake_npm)
+
+    entry = store.entry("fetch-" + "a" * 64)
+    entry.mkdir(parents=True)
+    archive = entry / "npm-1.0.0.tgz"
+    archive.write_bytes(b"archive")
+    staged = tmp_path / "scratch" / "tree"
+
+    get_package("npm").unpack(archive, staged, target)
+
+    assert [p.name for p in entry.iterdir()] == [archive.name]
+    assert caches and not caches[0].is_relative_to(staged)