Motivation for this change

There is an open discussion about how to add ImageMagick-support to emacs. Unfortunately, there is no solution to add ImageMagick-support to emacs when using EXWM. This PR is an intent to fix this.

I added pkgs.imagemagick to the cfg.extraPackages attribute in my config (though I'm not sure if it's really necessary).

This is just a proposal, and I see that, for now, the code is not very flexible. I'm happy for suggestions on how to make adding packages to exwm's emacs more flexible, in case it's necessary.

Closes #70631

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.