ci: remove redundant pnpm workspace install from Playwright test jobs

The test scripts already run their own `pnpm install` in
packages/buckaroo-js-core/. The CI-level workspace-wide
`pnpm install --frozen-lockfile` was redundant. pnpm setup and cache
are kept so the scripts' install is fast.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: paddymul

.github/workflows/checks.yml

@@ -254,14 +254,11 @@ jobs:
         uses: pnpm/action-setup@v4
         with:
           version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           cache: 'pnpm'
           cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with:
@@ -294,14 +291,11 @@ jobs:
         uses: pnpm/action-setup@v4
         with:
           version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           cache: 'pnpm'
           cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with:
@@ -368,14 +362,11 @@ jobs:
         uses: pnpm/action-setup@v4
         with:
           version: 9.10.0
-      - name: Setup Node.js with pnpm cache
+      - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           cache: 'pnpm'
           cache-dependency-path: 'packages/pnpm-lock.yaml'
-      - name: Install pnpm dependencies
-        working-directory: packages
-        run: pnpm install --frozen-lockfile
       - name: Download build artifacts
         uses: actions/download-artifact@v4
         with: