Add cache to yarn install in actions

.github/workflows/validate.yml

@@ -1,13 +1,6 @@
-name: Nx validate CI
+name: Validate
 
-on:
-  push:
-    branches:
-      - master
-      - dev
-  pull_request:
-    branches:
-      - "*"
+on: pull_request
 
 jobs:
   validate:
@@ -16,8 +9,18 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
         with:
-          node-version: "14"
-      - run: yarn
+          node-version: "14.x"
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+      - uses: actions/cache@v1
+        id: yarn-cache
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+      - run: yarn --prefer-offline
       - run: yarn db-gen
       - run: yarn nx lint
       - run: yarn nx test