nullmailer: fix relative -> absolute path in preStart script

(cherry picked from commit f9d64a068b90f196d43b91c81311547016b75fe9)

nixos/modules/services/mail/nullmailer.nix

···
 
       preStart = ''
         mkdir -p /var/spool/nullmailer/{queue,tmp}
-        rm -f var/spool/nullmailer/trigger && mkfifo -m 660 /var/spool/nullmailer/trigger
+        rm -f /var/spool/nullmailer/trigger && mkfifo -m 660 /var/spool/nullmailer/trigger
         chown ${cfg.user} /var/spool/nullmailer/*
       '';