export: Actually fix XDG_DATA_DIRS and XDG_DATA_HOME handling

[TigerGorilla2]

This fixes the use of XDG_DATA_DIRS and XDG_DATA_HOME in accordance to the specification.
As per the specification

There is a set of preference ordered base directories relative to which data files should be searched. This set of directories is defined by the environment variable $XDG_DATA_DIRS.

and

There is a single base directory relative to which user-specific data files should be written. This directory is defined by the environment variable $XDG_DATA_HOME.

This change now uses the correct fallbacks for each variable (was swapped) and does not try to iterate over XDG_DATA_HOME as it should only contain a single directory.

[89luca89]

Thanks @TigerGorilla2 LGTM!