go-mtpfs: fix darwin build

pkgs/tools/filesystems/go-mtpfs/default.nix

···
     sha256 = "1jcqp9n8fd9psfsnhfj6w97yp0zmyxplsig8pyp2gqzh4lnb5fqm";
   };
 
+  preFixup = stdenv.lib.optionalString stdenv.isDarwin ''
+    install_name_tool -delete_rpath "$out/lib" $bin/bin/go-mtpfs
+  '';
+
   goDeps = ./deps.nix;
 }