Build .deb containing briar-headless.jar

Since packaging briar-headless in Debian main (briar-debian#1) is currently blocked, efforts will now focus on creating an official APT repository by The Briar Project (briar-debian#3). The briar-headless.deb provided in that repository will ideally come from briar-reproducer. We need to extend briar-reproducer to not only create a briar-headless.jar, but also a briar-headless.deb that contains the .jar.