fix: v2.5.1 — plugin npm runs no longer depend on PATH (Windows update failure)

A Windows self-update failed at "bootstrap": 'npm.cmd' is not recognized.
The update chain runs with the dashboard server's environment, whose PATH
need not reach npm.cmd, and the bootstrap looked npm up through cmd.exe
(shell: true, which also printed DEP0190).

- npmCommand() in scripts/plugin-bootstrap.js runs the npm bundled with the
  running node (npm-cli.js beside process.execPath, Windows and Unix
  layouts) with no shell, and puts that node's directory first on PATH (kept
  under the env's own Path/PATH key) so npm scripts find the same node; npm
  on PATH only when that node ships none
- Used by the runtime install, the UI build (plugin-open.js) and the
  doctor's npm row (plugin-doctor.js)

scripts/plugin-doctor.js

@@ -125,10 +125,9 @@ function runtimeDepsStatus(rt) {
 
 /** No npm, no dashboard bundle and no runtime install — both go through it. */
 function npmStatus() {
-  const res = spawnSync(process.platform === "win32" ? "npm.cmd" : "npm", ["-v"], {
-    encoding: "utf8",
-    shell: process.platform === "win32",
-  });
+  // The same npm the bootstrap and the UI build run (`npmCommand`), not PATH's.
+  const npm = boot.npmCommand(["-v"]);
+  const res = spawnSync(npm.file, npm.args, { encoding: "utf8", env: npm.env, shell: npm.shell });
   if (res.status !== 0) {
     return ["fail", "npm", "not runnable — the runtime install and the UI build both need it"];
   }