mlton: disable parallel build

pkgs/development/compilers/mlton/from-git-source.nix

@@ -21,6 +21,9 @@ stdenv.mkDerivation {
 
   buildInputs = [mltonBootstrap gmp];
 
+  # build fails otherwise
+  enableParallelBuilding = false;
+
   preBuild = ''
     find . -type f | grep -v -e '\.tgz''$' | xargs sed -i "s@/usr/bin/env bash@$(type -p bash)@"
     sed -i "s|/tmp|$TMPDIR|" bin/regression