CI: Use checkout action itself for initializing submodules

.github/workflows/main.yml

@@ -12,13 +12,14 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - name: Checkout repo
+        uses: actions/checkout@v2
+        with:
+          submodules: true
       - name: set up JDK 1.8
         uses: actions/setup-java@v1
         with:
           java-version: 1.8
-      - name: Initialize submodules
-        run: git submodule update --init
       - name: Grant execute permission for gradlew
         run: chmod +x gradlew
       - name: Build with Gradle