ci: fix pnpm install (#51)

.github/workflows/test.yml

@@ -9,6 +9,17 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
-      - run: npm i pnpm@latest -g
+      - name: Cache pnpm modules
+        uses: actions/cache@v2
+        env:
+          cache-name: cache-pnpm-modules
+        with:
+          path: ~/.pnpm-store
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ matrix.node-version }}-${{ hashFiles('**/package.json') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-${{ matrix.node-version }}-
+      - uses: pnpm/action-setup@v1.2.1
+        with:
+          version: latest
       - run: pnpm install
       - run: npm test