try adding display via xvfb

Author: heuermh

.github/workflows/ci.yml

@@ -17,4 +17,7 @@ jobs:
           path: ~/.m2
           key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
           restore-keys: ${{ runner.os }}-m2
-      - run: mvn --batch-mode --update-snapshots clean package
+      - run: |
+          export DISPLAY=:99
+          sudo Xvfb -ac :99 -screen 0 1280x1024x24 > /dev/null 2>&1 &
+          mvn --batch-mode --update-snapshots clean package