Successful container-build causes artifact warnings

Added in 7ea69605, we generate container-build-report.xml on failure only but add it unconditionally to the artifacts. This causes warnings for successful builds:

Uploading artifacts for successful job
00:02
Uploading artifacts...
WARNING: container-build-report.xml: no matching files 
ERROR: No files to upload                          
Uploading artifacts...
WARNING: container-build-report.xml: no matching files 
ERROR: No files to upload    

See e.g. this job.

The simplest solution:

artifacts:
  when: on_failure
  ...

doesn't work because that's inherited to the children. Can be overridden there I supposed but it changes the expected default behaviour and will cause frustration, so let's not do that.

Found by @bentiss, I'm just filing this so we don't forget about it. cc @jadahl