chore: explicitly install pnpm

Author: uetchy

.github/workflows/test.yml

@@ -19,6 +19,10 @@ jobs:
         with:
           node-version: ${{ matrix.node }}
 
+      - uses: pnpm/[email protected]
+        with:
+          version: 6.0.2
+
       - name: Install dependencies
         run: yarn install