drbd: fix evaluation after #30872

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

···
 
   preConfigure =
     ''
-      export PATH=${systemd.udev.bin}/sbin:$PATH
+      export PATH=${systemd}/sbin:$PATH
       substituteInPlace user/Makefile.in \
         --replace /sbin '$(sbindir)'
       substituteInPlace user/legacy/Makefile.in \