Wire backup_borg_username

It's probably unnecessary, as this user is only used in the borg container
internally, but.. It doesn't hurt to set it to `matrix`.

group_vars/matrix_servers

@@ -1882,6 +1882,7 @@ backup_borg_enabled: false
 
 backup_borg_identifier: matrix-backup-borg
 
+backup_borg_username: "{{ matrix_user_username }}"
 backup_borg_uid: "{{ matrix_user_uid }}"
 backup_borg_gid: "{{ matrix_user_gid }}"