Change the $phpbb_adm_relative_path is not supported by the installer

Hi,

In config.php, If we change the value of $phpbb_adm_relative_path , the installation system is not able to load the template files.

As on line 119 in /install/startup.php, the admin_path will never be set, the admin_path will always be set with adm/

To fix that point, i suggest to add the following code before the line 119

$phpbb_config_php_file = new \phpbb\config_php_file($phpbb_root_path, $phpEx);

extract($phpbb_config_php_file->get_all());

After adding this code, this will allow to load the template.

But the CSS fails to be loaded, because adm/ is hardcoded in phpbb\install\controller\helper.php on line 124

For this point, I don't know how to fix it.

Thanks in advance for taking this bug into account.
Regards.

— Edit —

Another instance of adm/ located on line 260 in phpbb/installer/install.php

Maybe adm/ is present in other place.