Why must GUIX_LOCPATH be manually exported?

[Greg Hogan]

I am seeing the following warning running Guix on a foreign distribution:

    $ man man
    man: can't set the locale; make sure $LC_* and $LANG are correct

The errors disappear when I manually configure GUIX_LOCPATH, as
described at 
https://guix.gnu.org/manual/devel/en/html_node/Application-Setup.html:

    $ guix install glibc-locales
    $ export GUIX_LOCPATH=$HOME/.guix-profile/lib/locale

Why is GUIX_LOCPATH not automatically added to the user profile
(loaded with 'source ${GUIX_PROFILE}/etc/profile')? We see with `guix
shell` that GUIX_LOCPATH is not set in the user environment:

    $ guix shell glibc-locales --pure --search-paths
    <no output>

If GUIX_LOCPATH cannot be automatically exported in the user profile,
I am wondering if it could be set from the manifest by the example
custom 'my-glibc-locales' from the link above.

Greg