Add caching to workflow.

.github/workflows/gradle.yml

@@ -22,5 +22,12 @@ jobs:
         java-version: 1.8
     - name: Grant execute permission for gradlew
       run: chmod +x gradlew
+    - uses: actions/cache@v2
+      with:
+        path: |
+          ~/.gradle/caches
+          ~/.gradle/wrapper
+        key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
+        restore-keys: ${{ runner.os }}-gradle-
     - name: Build with Gradle
       run: ./gradlew build