filezilla: add wrapper to set necessary FZ_DATADIR

Filezilla fails to start currently, with the following message being
shown in a lone dialog before the program exits:

> Could not find the resource files for FileZilla, closing FileZilla.
> You can set the data directory of FileZilla using the '--datadir
> <custompath>' commandline option or by setting the FZ_DATADIR
> environment variable.

This commit adds a wrapper for the filezilla binary to set the
environment variable to the package root, which fixes the issue.