CI: add an upload-artifact action step

Author: carlosame

.github/workflows/build.yml

@@ -46,3 +46,14 @@ jobs:
       # Build
       - name: Build with Gradle
         run: ./gradlew build
+
+      # Upload test artifact
+      - name: Upload test output
+        uses: actions/upload-artifact@v6
+        if: ${{ failure() }}
+        with:
+          name: test-output-${{ matrix.java }}
+          path: |
+            build/reports/
+          retention-days: 15
+          overwrite: true