Correct zerotier role to be more clear about apt-centricity

roles/zerotier/tasks/main.yml

@@ -1,14 +1,17 @@
 #!/usr/bin/env ansible-playbook
 # vim:ft=ansible:
 ---
-- name: ensure zerotier repo key
+- name: configure zerotier for apt
+  block:
+    - name: ensure zerotier repo key
       apt_key: url=https://raw.githubusercontent.com/zerotier/ZeroTierOne/master/doc/contact%40zerotier.com.gpg
-- name: ensure zerotier repo
+    - name: ensure zerotier repo
       apt_repository: repo="deb http://download.zerotier.com/debian/buster buster main"
-- name: update apt cache
+    - name: update apt cache
       apt: update_cache=yes cache_valid_time=86400
-- name: ensure packages
+    - name: ensure packages
       apt: name=zerotier-one
+  when: ansible_pkg_mgr == "apt"
 - name: template unit file
   template: src=zerotier-one.service dest=/etc/systemd/system/zerotier-one.service
   notify: restart zerotier