fix(ci): add pnpm installation step

Author: Alex K

.github/workflows/ci.yml

@@ -18,6 +18,9 @@ jobs:
           node-version: 20
           cache: 'pnpm'
 
+      - name: Install pnpm
+        run: npm install -g pnpm
+
       - name: Install dependencies
         run: pnpm install