haskell.compiler.ghcHEAD: provide xattr on darwin

This darwin specific tool is required for GHC on master
nowadays. Unfortunately the XATTR environment variable only works in the
GHC 8.10 series.

pkgs/development/compilers/ghc/head.nix

@@ -2,7 +2,7 @@
 
 # build-tools
 , bootPkgs
-, autoconf, autoreconfHook, automake, coreutils, fetchgit, perl, python3, m4, sphinx
+, autoconf, autoreconfHook, automake, coreutils, fetchgit, perl, python3, m4, sphinx, xattr
 , bash
 
 , libiconv ? null, ncurses
@@ -255,6 +255,10 @@ stdenv.mkDerivation (rec {
     ghc bootPkgs.alex bootPkgs.happy bootPkgs.hscolour
   ] ++ lib.optionals enableDocs [
     sphinx
+  ] ++ lib.optionals stdenv.isDarwin [
+    # TODO(@sternenseemann): use XATTR env var once we have
+    # https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6447
+    xattr
   ];
 
   # For building runtime libs

pkgs/top-level/haskell-packages.nix

@@ -126,6 +126,10 @@ in {
     ghcHEAD = callPackage ../development/compilers/ghc/head.nix {
       bootPkgs = packages.ghc901; # no binary yet
       inherit (buildPackages.python3Packages) sphinx;
+      # Need to use apple's patched xattr until
+      # https://github.com/xattr/xattr/issues/44 and
+      # https://github.com/xattr/xattr/issues/55 are solved.
+      inherit (buildPackages.darwin) xattr;
       buildLlvmPackages = buildPackages.llvmPackages_10;
       llvmPackages = pkgs.llvmPackages_10;
       libffi = pkgs.libffi;