Add cache

.github/workflows/maven.yml

@@ -12,9 +12,14 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-
     steps:
     - uses: actions/checkout@v2
+    - name: Cache Maven packages
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2
     - name: Checkout submodules
       uses: textbook/git-checkout-submodule-action@2.1.1
     - name: Set up JDK 16