The CI cannot find screenshot in the case of failing environment tests anymore

XWIKI-24178 changed a bit where screenshots are located in case the system property "maven.build.dir" is set, but the pipeline cannot use the API and was not updated to take that into account.

That being said, there is no reason for the screenshots folder to be based on getOutputDirectory() and it should be based on getMavenBuildDirectory() (which is environment's build expectation).

It will still require modifying the pipeline since the main build will become wrong (since the logic for case where "maven.build.dir" is set and cases where "maven.build.dir" is not set was completly different before and is now consistent).