services: mcron: Log to a file.

This makes it easier to read the output, as it's recorded in a file.

* gnu/services/mcron.scm (mcron-shepherd-services): Add #:log-file to
make-forkexec-constructor.

gnu/services/mcron.scm

@@ -121,7 +121,9 @@ (define mcron-shepherd-services
                         (cons* "GUILE_AUTO_COMPILE=0"
                                "PATH=/run/current-system/profile/bin"
                                (remove (cut string-prefix? "PATH=" <>)
-                                       (environ)))))
+                                       (environ)))
+
+                        #:log-file "/var/log/mcron.log"))
               (stop #~(make-kill-destructor))
 
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