home: shepherd: "shepherd.conf" returns one value.

Fixes <https://issues.guix.gnu.org/54779>.
Reported by Aleksandr Vityazev <avityazev@posteo.org>
and Nicolas Graves <ngraves@ngraves.fr>.

* gnu/home/services/shepherd.scm (home-shepherd-configuration-file):
Return one value.

gnu/home/services/shepherd.scm

@@ -84,7 +84,10 @@ (define config
                                         services))))
             (if (defined? 'start-in-the-background)
                 (start-in-the-background services-to-start)
-                (for-each start services-to-start)))))
+                (for-each start services-to-start))
+
+            ;; Return one value.
+            #t)))
 
     (scheme-file "shepherd.conf" config)))