ci(coverage): gitlab-ci is able to read the coverage report we produce

.gitlab-ci.yml

@@ -126,6 +126,7 @@ py3-server:
       - /reports/coverage-server.xml
     reports:
       junit: /builds/cubicweb/cubicweb/server.xml
+      cobertura: /builds/cubicweb/cubicweb/reports/coverage-server.xml
     expire_in: 2 weeks
 
 py3-web:
@@ -154,6 +155,7 @@ py3-web:
       - /reports/coverage-web.xml
     reports:
       junit: /builds/cubicweb/cubicweb/web.xml
+      cobertura: /builds/cubicweb/cubicweb/reports/coverage-web.xml
     expire_in: 2 weeks
 
 py3-misc:
@@ -182,6 +184,7 @@ py3-misc:
       - /reports/coverage-misc.xml
     reports:
       junit: /builds/cubicweb/cubicweb/misc.xml
+      cobertura: /builds/cubicweb/cubicweb/reports/coverage-misc.xml
     expire_in: 2 weeks
 
 test-cube-skeleton: