feat: add cache for CI (conwnet#182)

Author: xcv58

.github/workflows/build.yml

@@ -22,6 +22,18 @@ jobs:
         uses: actions/setup-node@v1
         with:
           node-version: ${{ matrix.node-version }}
+
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+
+      - uses: actions/cache@v2
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}-${{ matrix.node-version }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}-${{ matrix.node-version }}
+
       - run: yarn --frozen-lockfile
       - run: yarn eslint
       - run: yarn build