CI: cache yarn directory

Author: th-ch

.github/workflows/build.yml

@@ -20,6 +20,18 @@ jobs:
         with:
           node-version: "12.x"
 
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+
+      - uses: actions/cache@v2
+        id: yarn-cache
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+
       - name: Install dependencies
         run: yarn --frozen-lockfile