fish: how to change the vendor_* directories

[ng0]

Today I noticed that my fish completions don't work if you install them from
within guix.
They do work if they are in one of the scanned directories, which includes and 
used to be
~/.config/fish/completions/ where I symlinked my developer copy to.
So I just learned about ${PREFIX}/share/fish/vendor_functions.d being one of 
the directories
which is intended for third party addons.
We can not change fish to pick up 
$HOME/.guix-profile/share/fish/vendor_functions.d, at least
not from what I tried in 5 minutes. Vim was easier, you are able to set a path 
manually.
Any suggestions on how we can hack this? Maybe simply extend the SEARCH PATH?
-- 
https://pragmatique.xyz
PGP: https://people.pragmatique.xyz/ng0/