Motivation for this change

when trying to fix the plasma tests to fix #98657 (comment) I stumbled onto the fact that all plasma tests are broken in master right now

git bisect'd it to c451603

this is because the nixos tests don't allow for unfree fonts.
example error:

$ nix-build -A nixosTests.plasma5
error: Package ‘font-bh-lucidatypewriter-100dpi-1.0.3’ in /home/jon/projects/nixpkgs/pkgs/servers/x11/xorg/default.nix:199 has an unfree license (‘unfree’), refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.

(use '--show-trace' to show detailed location information)

This PR is just a "quick fix". not sure what ramification this will have normal users

broken tests:

[19:42:41] jon@nixos ~/projects/nixpkgs/nixos/tests (fix-nixos-tests)
$ rg xserver -l
lightdm.nix
xfce.nix
xautolock.nix
xmonad.nix
pt2-clone.nix
signal-desktop.nix
networking-proxy.nix
gnome3.nix
gnome3-xorg.nix
i3wm.nix
common/auto.nix
common/x11.nix
plasma5.nix
enlightenment.nix
pantheon.nix
keymap.nix
shattered-pixel-dungeon.nix
sddm.nix
virtualbox.nix

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.