mdadm: fix path to sendmail

Without this, mdadm won't be able to send email notifications:

  $ sudo mdadm --monitor --scan --test
  sh: /nix/store/2v8jn0lxza72grcm6hciak9fpgm7xb3a-system-sendmail-1.0: Is a directory

Fixes: b074a40f ("mdadm: use shared system-sendmail")