roles/common: Don't overwrite spamhaus nft sets

The ones in this repo are only placeholders that get updated by the
update-spamhaus-nftables service, so we shouldn't overwrite them if
they exist.

roles/common/tasks/firewall_Debian.yml

@@ -39,7 +39,7 @@
 
   - name: Copy extra nftables configuration files
     when: ansible_distribution_major_version is version('11', '>=')
-    copy: src={{ item }} dest=/etc/nftables/{{ item }} owner=root group=root mode=0644
+    copy: src={{ item }} dest=/etc/nftables/{{ item }} owner=root group=root mode=0644 force=no
     loop:
       - spamhaus-ipv4.nft
       - spamhaus-ipv6.nft