cache mvn repo in GH build

Signed-off-by: Lukas Jungmann <lukas.jungmann@oracle.com>

.github/workflows/maven.yml

diff --git a/.github/workflows/maven.yml b/.github/workflows/maven.yml
index d64dcfb..61895ed 100644
--- a/.github/workflows/maven.yml
+++ b/.github/workflows/maven.yml
@@ -26,6 +26,13 @@
         java_version: [ 11, 17-ea ]
 
     steps:
+    - name: Cache local Maven repository
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2/repository
+        key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+        restore-keys: |
+          ${{ runner.os }}-maven-
     - name: Checkout for build
       uses: actions/checkout@v2.3.4
     - name: Set up JDK