fix GitHub action workflow for pnpm

Author: nlordell

.github/workflows/ci.yaml

@@ -9,10 +9,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 9
       - uses: actions/setup-node@v4
         with:
           node-version: 20.x
-          cache: yarn
-          cache-dependency-path: yarn.lock
-      - run: yarn
-      - run: yarn build
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile