Add Maven build/dependency caching

Author: C0urante

.github/workflows/pull_request.yml

@@ -15,6 +15,7 @@ jobs:
         uses: actions/setup-java@v1
         with:
           java-version: 8
+          cache: maven
       - name: Build (Maven)
         run: mvn --batch-mode clean package -DskipTests
       - name: Unit tests (Maven)