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 }}"
 

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -95,5 +95,3 @@
           Please change your configuration (vars.yml) to rename all variables (`matrix_backup_borg_` -> `backup_borg_`).
           We found usage of the following variables: {{ matrix_playbook_migration_backup_borg_migration_vars.keys() | join(', ') }}
       when: "matrix_playbook_migration_backup_borg_migration_vars | length > 0"
-
-