scan-licenses: Only fail in target on unknown licenses

Previously ci-license-scan failed if it was not able to detect the license of some files, even if are no licensing restrictions in the component that hosts the package.

This was annoying because it caused the scan to block pipelines where there was little value in adding the missing licensing information: for instance, while it is important on target, it is not particularly relevant everywhere else.

Recently the --fail-on-unknown flag has been added to ci-license-scan and it now defaults to not failing unless it is set.

To realign things, pass --fail-on-unknown when a blacklist is set, which means we actually care about licenses being detected.