enable yarn cache, hopefully

.github/workflows/codechecks.js.yml

@@ -19,15 +19,26 @@ jobs:
         node-version: [14.x]
 
     steps:
-    - name: Checkout project source
+    - name: Checkout source
       uses: actions/checkout@v2
     - name: Install tools
       uses: actions/setup-node@v2
       with:
         node-version: ${{ matrix.node-version }}
-    - name: Install project dependencies
-      run: npm install --also=dev
+    # See: https://github.com/actions/cache/blob/main/examples.md#node---yarn
+    - name: Get Yarn cache directory
+      id: yarn-cache-dir-path
+      run: echo "::set-output name=dir::$(yarn cache dir)"
+
+    - name: Use Yarn cache
+      uses: actions/cache@v2
+      id: yarn-cache
+      with:
+        path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+        key: ${{ runner.os }}-yarn-${{ matrix.node-version }}-${{ hashFiles('**/yarn.lock') }}
+    - name: Install dependencies
+      run: yarn install --prefer-offline --frozen-lockfile
     - name: Unit tests
-      run: npm test
+      run: yarn test
     - name: Lint
-      run: npm run lint-ci
+      run: yarn run lint-ci