Declare project.build.outputTimestamp to make the build reproducible

Are you sure we really need to ? I'm wondering if the command line -Dproject.build.outputTimestamp would not have the same effect.

Then how do you choose this value? The value of specifying the value in the pom is that anybody who has access to the source repository can rebuild every release bit-by-bit as the value is included in the source. If we don't specify it in the source, we would need to document with every release which value we used and it would be much harder to automatically verify that XWiki is actually reproducible. The value in the source is also automatically set to the current date and time when the version is set during the release while we would need to manually implement a strategy how to increment the value we specify during every release.