The package manager is intended to install packages in the apps vhost and runs under the permissions of the ispapps user and group. This decision has been made for security reasons. The apps vhost root directory is in /var/www/apps/
The problem with your package is that you try to access directories outside of the apps vhost. Also a path like /usr/share/roundcube would collide with the packages of the debian system, so I would not install a custom software package there as a simple "apt-get insall roundcube" would destrpy your installation on debian and ubuntu.
My recommendation is that you install roundcube to /var/www/apps/roundcube. The application will then run safely under the ispapps user and group and can be accessed on port 8081.
I will check how we can extend the apps installer to create a /webmail alias automatically.