MantisBT install page included instead of plugin-specific install functions

Created by: dregad

When the plugin's schema function executes, the current working directory is that of the parent script, i.e. the one which calls plugin_needs_upgrade(), usually via plugin_init_installed() in core.php.

When said parent script is one of MantisBT's admin scripts, typically the index page, this results in MantisBT's install.php file to be included instead of the plugin's, resulting in the page displaying a (non-functional) install page on top of its actual contents as shown in the screenshot below

Regression introduced in 2.3.0 by 20484c13.