thunar-with-plugins: fix wrapper meta inheritance

This was an edge case that #400458 missed, as there was no
reason to evaluate NixOS configurations during the review process.

This fixes the meta attribute inheritance in the wrapped version of
`xfce.thunar`, which would only be called by a NixOS module, and does
not appear in the main package set.

Reported-by: stormytuna <stormytuna@outlook.com>
Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>