ci: Fix paths shown in the website

ci: Fix paths shown in the website

Right now we're setting the prefix to a custom path, which
results in paths like

  /builds/libvirt/libvirt/vroot/etc/libvirt/virtqemud.conf

ending up in the generated HTML. In order to avoid that,
set the prefix and other installation paths to reasonable
default values by passing

  -Dsystem=true

and then take advantage of $DESTDIR support to still be able
to write the HTML files without requiring root privileges.

Reported-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>