bug#42157: guix-install.sh set XDG_DATA_DIRS before sourcing profile

[Fredrik Salomonsson]

Hi,

I encountered an issue when installing guix using the installer script
on Arch Linux. I'm using a quite simple setup, with only SDDM and sway,
so no XDG environment variables are set.

Noticed after I've installed guix, things started braking. I tracked it
down to the `XDG_DATA_DIRS` environment variable only contained paths to
the guix store.

The guix.sh script does set `XDG_DATA_DIRS` to
/usr/local/share/:/usr/share/ if it's not defined. But it does that
after it has sourced the `$GUIX_PROFILE/etc/profile`. Which for me adds
some paths to the `XDG_DATA_DIRS` and therefore the default paths are
not added.

I worked around the issue by adding a script to the /etc/profile.d that
sets the `XDG_DATA_DIRS` to `/usr/local/share/:/usr/share/` and made
sure it got loaded before the guix.sh script (named it 00-xdg.sh).

But might be good to make sure `XDG_DATA_DIRS` is set to the default
paths before it sources the profile. To avoid this issue for other
users.

Thanks

-- 
s/Fred[re]+i[ck]+/Fredrik/g