guix system: Fix mistaken 'guix pull' warning upon 'reconfigure'.

Fixes <https://bugs.gnu.org/38196>.
Reported by Florian Pelz <pelzflorian@pelzflorian.de>.

* guix/scripts/system.scm (maybe-suggest-running-guix-pull): Check
whether 'current-profile' returns true instead of checking for the
existence of ~root/.config/guix/current.  That way, "sudo guix system
reconfigure" no longer emits a warning in that case.