ci: add packageManager field to pin pnpm version

Pins pnpm@10.32.1 via the packageManager field. This allows
pnpm/action-setup@v4 to resolve the version from package.json instead
of relying on `version: latest`, preventing silent breakage on major
pnpm version bumps.

Fixes: PRI-674

Co-Authored-By: Paperclip <noreply@paperclip.ing>

package.json

@@ -12,6 +12,7 @@
   "homepage": "https://github.com/privilegedescalation/headlamp-polaris-plugin#readme",
   "author": "privilegedescalation",
   "license": "Apache-2.0",
+  "packageManager": "pnpm@10.32.1",
   "scripts": {
     "start": "headlamp-plugin start",
     "build": "headlamp-plugin build",