Use paccache.service environment file to pass extra arguments

Use the new `/etc/conf.d/pacman-contrib` environment file (introduced in pacman/pacman-contrib!53) to pass extra arguments to `paccache.service` instead of overriding it completely.

This improves security as it allows to benefit from all the hardening implemented in the upstream service file (which wasn't the case previously, since the whole service file was overwriten).

/!\ Requires `pacman-contrib` >= `1.11.0` /!\

Closes #649

roles/common/files/paccache.service

-[Unit]
-Description=Discard unused packages
-
-[Service]
-Type=oneshot
-ExecStart=/usr/bin/paccache -r -c /var/cache/pacman/pkg

roles/common/tasks/main.yml

@@ -145,8 +145,11 @@
 - name: Install pacman-contrib for paccache/pacdiff and fakeroot for checkupdates
   pacman: name=pacman-contrib,fakeroot state=installed
 
-- name: Install custom paccache.service
-  copy: src=paccache.service dest=/etc/systemd/system/paccache.service owner=root group=root mode=0644
+- name: Add extra arguments in paccache.service environment file
+  lineinfile:
+    path: /etc/conf.d/pacman-contrib
+    regexp: '^PACCACHE_ARGS='
+    line: 'PACCACHE_ARGS=-c /var/cache/pacman/pkg'
 
 - name: Start and enable paccache timer
   systemd_service: name=paccache.timer enabled=yes state=started daemon_reload=yes