Add pnpm setup to GitHub Actions workflow

Author: fern-support

.github/workflows/preview-sdks.yml

@@ -17,6 +17,11 @@ jobs:
       - name: Setup node
         uses: actions/setup-node@v3
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v2
+        with:
+          version: 8
+
       - name: Download Fern
         run: npm install -g fern-api