nixos/services/kmscon: fix systemd configuration

[jflanglois]

Motivation for this change

This fixes #112616. The solution here follows the approach documented in the Arch Linux wiki.

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.

[xaverdh]

This will make kmscon run on every new tty right? Could we also add an option to the module so it only runs on certain ttys?

[jflanglois]

This will make kmscon run on every new tty right? Could we also add an option to the module so it only runs on certain ttys?

That is correct, but that is also the intent of the existing code (unless I misunderstand). I would suggest we expand that in a follow up PR since this one is just meant to fix the broken module.

[symphorien]

I reproduced the issue on a nixos-rebuild build-vm vm without the PR and checked it is fixed with it.

[jflanglois]

Thank you!