ci: activate pnpm via corepack (action-setup installer fails on self-hosted)

pnpm/action-setup's installer fails on the containerized runner even in
standalone mode (Cannot find module '../lib/cli.js'). corepack ships with
node 22/24, which setup-node installs fine on arm64. Dropped the GitHub
pnpm cache too — the runner image persists the pnpm store on a shared
named volume at pnpm's default store path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: nficano

.github/workflows/test.yml

@@ -49,22 +49,21 @@ jobs:
         with:
           fetch-depth: 1
 
-      - name: Setup pnpm
-        # pnpm/action-setup v4.0.0
-        uses: pnpm/action-setup@0e279bb959325dab635dd2c09392533439d90093 # v4.0.0
-        with:
-          version: 9.15.0
-          run_install: false
-          # The default installer needs a system node at install time, which
-          # the containerized self-hosted runner doesn't have. The standalone
-          # binary is dependency-free (linux-arm64 builds available).
-          standalone: true
-
       - name: Setup Node.js ${{ matrix.node }}
         uses: actions/setup-node@v6
         with:
           node-version: ${{ matrix.node }}
-          cache: "pnpm"
+
+      # pnpm/action-setup's installer fails on the containerized self-hosted
+      # runner (no system node at install time; standalone mode also broken).
+      # corepack ships with node 22/24 and handles arm64 fine. The pnpm store
+      # persists across jobs on a shared named volume at the default store
+      # path, so no GitHub cache step is needed.
+      - name: Setup pnpm (via corepack)
+        run: |
+          corepack enable
+          corepack prepare pnpm@9.15.0 --activate
+          pnpm --version
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile