chore(install): stop writing distro-default config into system.nix

console.useXkbConfig and boot.initrd.systemd.enable are set distro-wide
(lib.mkDefault true in modules/nixos/default.nix, since the LUKS-keymap
fix), so the installer rewriting them in the generated system.nix was
redundant. Drop them there and from the offline-pin test fixture in
flake.nix — the nomarchy module still defaults them on, so the closure
(and the pin set) is unchanged; verified the values still resolve true
and `nix flake check` passes. Only the machine-specific xkb layout is
written now. Adjusts the README keyboard-option wording to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.nix

@@ -252,12 +252,6 @@
                       ({ lib, ... }: {
                         nomarchy.system.snapper.enable = true;
                         nomarchy.system.greeter.autoLogin = username;
-                        # The installer writes these (keyboard-everywhere
-                        # incl. the LUKS prompt) — mirror them or the
-                        # initrd/vconsole pins don't match an offline
-                        # install's rebuild set.
-                        console.useXkbConfig = true;
-                        boot.initrd.systemd.enable = true;
                         # nixos-generate-config keys microcode off
                         # enableRedistributableFirmware (now on): the
                         # real machine enables exactly one vendor —