mlton: add one-liner from old derivation to fix source files

Updates from-git-source.nix to use the one-liner to fix up the bash
shebang lines.

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

@@ -18,6 +18,8 @@ stdenv.mkDerivation {
   buildInputs = [mltonBootstrap gmp];
 
   preBuild = ''
+    find . -type f | grep -v -e '\.tgz''$' | xargs sed -i "s@/usr/bin/env bash@$(type -p bash)@"
+
     makeFlagsArray=(
       MLTON_VERSION="${version} ${rev}"
       CC="$(type -p cc)"