sane_backends: disable parallel install

parallel install creates a bad symlink at $out/lib/sane/libsane.so.1
which prevents finding plugins.

Fixes https://github.com/NixOS/nixpkgs/issues/224569

pkgs/applications/graphics/sane/backends/default.nix

···
   ''
   + lib.concatStrings (builtins.map installFirmware compatFirmware);
 
+  # parallel install creates a bad symlink at $out/lib/sane/libsane.so.1 which prevents finding plugins
+  # https://github.com/NixOS/nixpkgs/issues/224569
+  enableParallelInstalling = false;
+
   meta = with lib; {
     description = "SANE (Scanner Access Now Easy) backends";
     longDescription = ''