CI: Add step that validates the Gradle wrapper

Validates the gradle-wrapper.jar file on each build, which is a binary blob of executable code. This action ensures it's legit and doesn't execute malicious code.

See https://github.com/gradle/wrapper-validation-action

.github/workflows/build-app-workflow.yaml

@@ -6,5 +6,7 @@ jobs:
     steps:
       - name: Checkout the code
         uses: actions/checkout@v2
+      - name: Validate Gradle wrapper
+        uses: gradle/wrapper-validation-action@e2c57acffb2c9aa5a8dc6eda2bbae0b6e495bd4c
       - name: Build the app
         run: ./gradlew build