Cache gradle workflow (#260)

* Cache gradle workflow

* Cache wrapper

.github/workflows/gradle.yml

@@ -9,6 +9,13 @@ jobs:
     runs-on: "ubuntu-latest"
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/cache@v2
+        with:
+          path: |
+            ~/.gradle/wrapper
+            ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
+          restore-keys: ${{ runner.os }}-gradle-
       - uses: gradle/wrapper-validation-action@v1
       - name: Set up JDK 16
         uses: actions/setup-java@v2