Rely on <build.finalName> to determine the name of a XAR artifact

When building a XAR, we currently don't have the possibility to override the final name of the generated artifact. It is however possible using other plugins, such as the maven-jar-plugin or the maven-war-plugin with the property <finalName> defined in the <build> block of the POM.

This seems to come from : https://github.com/xwiki/xwiki-commons/blob/fdafc61a8273fd56dfe3a831c301b93f20fb1ecd/xwiki-commons-tools/xwiki-commons-tool-xar/xwiki-commons-tool-xar-plugin/src/main/java/org/xwiki/tool/xar/XARMojo.java#L112