[CI] Nix Package Build Checks

[xarvex]

It has already been on my mind, but #890 spurred some more thought surrounding some kind of workflow to ensure the Nix package can build and functions. One thing that needs to happen is that pythonRemoveDeps = true should be removed and replaced with something like the following:

# ...
{
  # ...
  pythonRemoveDeps = lib.optional (!withJXLSupport) [ "pillow_jxl" ];
  pythonRelaxDeps = [
    # packages here
  ];
  # ...
}

Setting pythonRemoveDeps the way I did was a mistake as it completely removed dependency checking taking place at all. Here, it can be set to only ignore the Pillow JXL plugin if support is disabled.

I take care of this as part of #886.

One concern about having a workflow is that authors not familiar with Nix or NixOS might become confused about this mysterious failure when working on a contribution. My suggestion is that the workflow should only run for commits and PRs to this “origin” repo. On failure with a PR, a short comment that tags me can be made by the workflow (peter-evans/create-or-update-comment), that way things can be taken care of when pulling is ready.

This is something I am working on and tinkering with, this issue only exists for tracking... and for reminding me.