fix(known_hosts): dig package does not install on Arch

The conditionnal on `ensure dig is available` does not work on Arch
since the `which` command does not exists. As the `pkg.installed`
state is idempotent, we don't need an extra check which depends on the
environment.

The `dig` utility is provided by `bind` on Arch and no more by
`bind-tools`.

openssh/known_hosts.sls

 ensure dig is available:
   pkg.installed:
     - name: {{ openssh.dig_pkg }}
-    - unless: which dig
 
 manage ssh_known_hosts file:
   file.managed:

openssh/osfamilymap.yaml

     server: openssh
     client: openssh
     service: sshd
-    dig_pkg: bind-tools
+    dig_pkg: bind
   sshd_config:
     Subsystem: sftp /usr/lib/ssh/sftp-server