Fix CI: pin pnpm version in GitHub Actions workflow

pnpm/action-setup@v4 requires an explicit version when packageManager
is not set in package.json.

Author: ThomasLohner

.github/workflows/ci.yml

@@ -17,6 +17,8 @@ jobs:
 
       - name: Install pnpm
         uses: pnpm/action-setup@v4
+        with:
+          version: 10
 
       - name: Install node
         uses: actions/setup-node@v6