bug#41666: $INFOPATH should end with a colon

[Jonas Bernoulli <jonas@bernoul.li>]

On a foreign distro Guix installs /etc/profile.d/guix.sh, which among
other things adds the directory where it installs info pages to the
INFOPATH variable.

It takes care to not accidentally cause the value to end with a colon.
Unfortunately it should do the opposite because a trailing colon means
"... and also the default locations", which is highly desirable.

This is documented in (info "(texinfo)Other Info Directories"):

>    However you set 'INFOPATH', if its last character is a colon (on
> MS-DOS/MS-Windows systems, use a semicolon instead), this is replaced by
> the default (compiled-in) path.  This gives you a way to augment the
> default path with new directories without having to list all the
> standard places.  For example (using 'sh' syntax):
>
>      INFOPATH=/home/bob/info:
>      export INFOPATH
>
> will search '/home/bob/info' first, then the standard directories.
> Leading or doubled colons are not treated specially.

I recommend you just make this simple change:

-  export INFOPATH="$_GUIX_PROFILE/share/info${INFOPATH:+:}$INFOPATH"
+  export INFOPATH="$_GUIX_PROFILE/share/info$:$INFOPATH"

If INFOPATH is already set and does not end with a colon, then that
should be left as-is; it might be intentional.