services: nar-herder: Specify a working directory.

Mostly so that core dumps end up here.

* gnu/services/guix.scm (nar-herder-shepherd-services): Specify a working
directory.

gnu/services/guix.scm

@@ -975,6 +975,7 @@ ca-certificates.crt file in the system profile."
                              '()))
                 #:user #$user
                 #:group #$group
+                #:directory "/var/lib/nar-herder"
                 #:pid-file "/var/run/nar-herder/pid"
                 #:environment-variables
                 `(,(string-append