guix-install.sh: Export INFOPATH to contain updated guix info-pages

* etc/guix-install.sh (sys_create_init_profile): Export INFOPATH to include
the updated info-pages from ~/.config/guix/current/share/info.  This also makes
sure that both /usr/bin/info and $GUIX_PROFILE/bin/info can read guix info pages
without throwing "no menu item 'guix' in node dir(Top)" error.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

etc/guix-install.sh

@@ -398,7 +398,12 @@ sys_create_init_profile()
     cat <<"EOF" > /etc/profile.d/guix.sh
 # _GUIX_PROFILE: `guix pull` profile
 _GUIX_PROFILE="$HOME/.config/guix/current"
-[ -L $_GUIX_PROFILE ] && export PATH="$_GUIX_PROFILE/bin${PATH:+:}$PATH"
+if [ -L $_GUIX_PROFILE ]; then
+  export PATH="$_GUIX_PROFILE/bin${PATH:+:}$PATH"
+  # Export INFOPATH so that the updated info pages can be found
+  # and read by both /usr/bin/info and/or $GUIX_PROFILE/bin/info
+  export INFOPATH="$_GUIX_PROFILE/share/info${INFOPATH:+:}$INFOPATH"
+fi
 
 # GUIX_PROFILE: User's default profile
 GUIX_PROFILE="$HOME/.guix-profile"