Fix display of coverage in Gitlab

This is the same fix that @thoennes made in walberla/walberla!573 (merged) and also the same as pystencils!310 (merged).

.gitlab-ci.yml

@@ -33,7 +33,9 @@ tests-and-coverage:
     paths:
       - coverage_report
     reports:
-      cobertura: coverage.xml
+      coverage_report:
+        coverage_format: cobertura
+        path: coverage.xml
       junit: report.xml
 
 # Normal test with longruns