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: b074a40f74 ("mdadm: use shared system-sendmail")

pkgs/os-specific/linux/mdadm/default.nix

@@ -28,7 +28,7 @@ stdenv.mkDerivation rec {
   postPatch = ''
     sed -e 's@/lib/udev@''${out}/lib/udev@' \
         -e 's@ -Werror @ @' \
-        -e 's@/usr/sbin/sendmail@${system-sendmail}@' -i Makefile
+        -e 's@/usr/sbin/sendmail@${system-sendmail}/bin/sendmail@' -i Makefile
     sed -i \
         -e 's@/usr/bin/basename@${coreutils}/bin/basename@g' \
         -e 's@BINDIR/blkid@${utillinux}/bin/blkid@g' \