Try yarn cache in github action

.github/workflows/main.yml

@@ -2,9 +2,9 @@ name: Node.js CI
 
 on:
   push:
-    branches: [ main ]
+    branches: [main]
   pull_request:
-    branches: [ main ]
+    branches: [main]
 
 jobs:
   build:
@@ -20,7 +20,20 @@ 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 config get cacheFolder)"
+
+      - uses: actions/cache@v2
+        id: yarn-cache # use this to check for `cache-hit` (`steps.yarn-cache.outputs.cache-hit != 'true'`)
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+
       - run: yarn install
       - run: yarn typecheck
       - run: yarn lint:all
-      - run: yarn test
+      - run: yarn test