itstool: fix double-shebang issue on macOS

pkgs/development/tools/misc/itstool/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, python3 }:
+{ stdenv, lib, fetchurl, python3 }:
 
 stdenv.mkDerivation rec {
   name = "itstool-2.0.6";
@@ -10,6 +10,22 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ (python3.withPackages(ps: with ps; [ libxml2 ])) ];
 
+  # bin/itstool's shebang is "#!${python3.withPackages(...)/bin/python} -s"
+  # withPackages' shebang is "#!#{bash}/bin/bash -e
+  #
+  # macOS won't allow the target of a shebang to be an interpreted script,
+  # causing bin/itstool to get interpreted as bash.
+  #
+  # By prefixing /usr/bin/env to the shebang, we have env fork/exec the python
+  # wrapper, which is perfectly happy to execute an interpreted script.
+  #
+  # However, we don't want to do this on Linux, which only allows one argument
+  # in a shebang.
+  postFixup = lib.optionalString stdenv.isDarwin ''
+    substituteInPlace $out/bin/itstool \
+      --replace "#!/" "#!/usr/bin/env /"
+  '';
+
   meta = {
     homepage = http://itstool.org/;
     description = "XML to PO and back again";