Motivation for this change

GNOME3 defaults to a US keyboard layout, with the user being able to configure this in the Settings dialog. Deleting the default value for keyboard layout makes it instead use the xkb default layout (source), which the NixOS manual says to set in configuration.nix (and is presumably where every other desktop environment / window manager gets their choice of keyboard layout from 😒)

GNOME's Wayland compositor still uses xkb, so this change doesn't depend on Xorg

I tried this change on my own system and found that GNOME had preserved the previous default setting for all of my users. I think this is probably ideal - current users see no change (some may not have set layout in configuration.nix, but only in gnome-control-center), whereas for new users the keyboard layout setting in configuration.nix works out-of-the-box (but gnome-control-center can still be used to change keyboard layout if desired)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.