ci: install pnpm in ng-renovate

The Angular CLI uses `pnpm` which is not pre-installed.

Author: alan-agius4

.github/workflows/ng-renovate.yml

@@ -16,6 +16,8 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - run: npm i -g pnpm
+        shell: bash
       - run: yarn --cwd .github/ng-renovate install --immutable
         shell: bash