pythonPackages.thumbor: 6.6.0 -> 6.7.0

This patch ensures that the currently broken `thumbor`[1] package builds
and works again.

The following problems were fixed:

* Rather than placing required packages (like `gifsicle` or `exiftool`)
  into the build input list, we reference them explicitly where needed
  to ensure that the package works after the build without further
  installs.

* Skip the `test_redeye_applied` test case which is broken for a while
  now.

[1] https://hydra.nixos.org/build/90290998