fix: improve borgbackup ssh keys handling

roles/ynh_backup/tasks/borgbackup.yml

@@ -56,7 +56,7 @@
 
     - name: deploy ssh private key for BorgBackup
       ansible.builtin.copy:
-        src: "{{ borg_ssh_keys_src }}"
+        src: "{{ borg_ssh_keys_src }}.vault"
         dest: "{{ borg_ssh_keys_dest }}"
         owner: "root"
         group: "root"