github actions: add wrapper validation (#875)

It validates the checksums of Gradle Wrapper JAR files present in the source tree and fails if unknown Gradle Wrapper JAR files are found. https://github.com/gradle/wrapper-validation-action

.github/workflows/build.yml

@@ -8,6 +8,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
+      - uses: gradle/wrapper-validation-action@v1
       - uses: actions/setup-java@v1
         with:
           java-version: ${{ matrix.java-version }}