enable Gradle wrapper validation

https://blog.gradle.org/gradle-wrapper-checksum-verification-github-action

Author: sullis

.github/workflows/ci.yml

@@ -51,6 +51,8 @@ jobs:
         platform: ['windows-latest', 'ubuntu-latest']
     steps:
       - uses: actions/checkout@v2
+      - name: Gradle wrapper validation
+        uses: gradle/wrapper-validation-action@v1
       - name: Set up JDK
         uses: actions/setup-java@v1
         with: