stack/erp5: don't produce HTML report for each test

The coverage of an individual test is after all not useful, only the
coverage of all tests combined is significant.
4 jobs for feat/coverage in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Coverage-Jerome

01:36:06

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Work

10:06:06

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Work

09:46:36

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Work

10:07:43