add pnpm for ember-assets job

Author: mansona

.github/workflows/ember-assets.yml

@@ -11,6 +11,7 @@ jobs:
     - uses: actions/checkout@v4
       with:
         fetch-depth: 0
+    - uses: pnpm/action-setup@v4
     - uses: mainmatter/ember-asset-size-action@v3
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"