[ticket/11536] Fixed incorrect removal of "install" in script_path

Earlier the script would incorrectly remove the word "install"
from the script_path when trying to remove the last folder in the
path named "/install". This would lead to issues when the path
you are installing phpBB contained "install". For example
"/install_test/install" would become "/_test".

This change gets the parent folder instead of replacing all
"install". $name contains /install_test/install/index.php
from start and running dirname() gives /install_test/install.
Running dirname once more gives the parent folder of install,
/install_test.

PHPBB3-11536