add action cache

Author: Nugine

Diff for: .github/workflows/deploy-demo.yml

@@ -15,6 +15,20 @@ jobs:
         with:
           node-version: '12'
 
+      # see https://github.com/actions/cache/blob/main/examples.md#node---yarn
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+
+      - name: Cache
+        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-
+
       - name: Install
         run: yarn install