Add unzip for diffoscope

The firefox diffoscope output requires unzip for making the diff
readable.

roles/rebuilderd_worker/tasks/main.yml

 - name: install required packages
-  pacman: name=rebuilderd,archlinux-repro,binutils state=present
+  pacman: name=rebuilderd,archlinux-repro,binutils,unzip state=present
 
 - name: configure rebuilderd-worker.conf
   template: src=rebuilderd-worker.conf.j2 dest=/etc/rebuilderd-worker.conf owner=rebuilderd group=rebuilderd mode=0660