Fix code coverage score

What does this MR do?

Fix badge for code coverage. Right now the coverage is unknown.

This regression was introduced in !289 (merged). See !289 (diffs, comment 904614689)

What are the relevant issue numbers?

No

Testing

Tested in https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/jobs/2305513021

The corbertura XML file is still exported.

total:														(statements)			80.1%
Uploading artifacts for successful job
00:02
Uploading artifacts...
coverage.xml: found 1 matching files and directories 
Uploading artifacts as "cobertura" to coordinator... 201 Created  id=2305513021 responseStatus=201 Created token=vjRYTjav
Cleaning up project directory and file based variables
00:01
Job succeeded