lightdm: fix removal of unused user data directories

This fixes errors such as

    Could not delete unused user data directory /var/lib/lightdm-data/.cache: Failed to execute child process “/bin/rm” (No such file or directory)
    Could not delete unused user data directory /var/lib/lightdm-data/.Xauthority: Failed to execute child process “/bin/rm” (No such file or directory)
    Could not delete unused user data directory /var/lib/lightdm-data/lightdm: Failed to execute child process “/bin/rm” (No such file or directory)

pkgs/applications/display-managers/lightdm/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, pam, pkgconfig, libxcb, glib, libXdmcp, itstool, libxml2
-, intltool, xlibsWrapper, libxklavier, libgcrypt, libaudit
+, intltool, xlibsWrapper, libxklavier, libgcrypt, libaudit, coreutils
 , qt4 ? null
 , withQt5 ? false, qtbase
 }:
@@ -36,6 +36,11 @@ stdenv.mkDerivation rec {
     "localstatedir=\${TMPDIR}"
   ];
 
+  prePatch = ''
+    substituteInPlace src/shared-data-manager.c \
+      --replace /bin/rm ${coreutils}/bin/rm
+  '';
+
   meta = {
     homepage = https://launchpad.net/lightdm;
     platforms = platforms.linux;