cosmic-greeter: add X11 xkb rules paths to wrapper

Set X11_BASE_RULES_XML and X11_BASE_EXTRA_RULES_XML environment variables
in the wrapper to fix xkeyboard_config path resolution. The xkb-data crate
used by cosmic-settings had hard-coded paths for the base.xml and extra.xml
files (/usr/share/X11/xkb/rules/), which are not compatible with NixOS.

pkgs/by-name/co/cosmic-greeter/package.nix

@@ -10,6 +10,7 @@
   linux-pam,
   udev,
   coreutils,
+  xkeyboard_config,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -60,6 +61,13 @@ rustPlatform.buildRustPackage (finalAttrs: {
     substituteInPlace src/greeter.rs --replace-fail '/usr/bin/env' '${lib.getExe' coreutils "env"}'
   '';
 
+  preFixup = ''
+    libcosmicAppWrapperArgs+=(
+      --set-default X11_BASE_RULES_XML ${xkeyboard_config}/share/X11/xkb/rules/base.xml
+      --set-default X11_BASE_EXTRA_RULES_XML ${xkeyboard_config}/share/X11/xkb/rules/extra.xml
+    )
+  '';
+
   meta = {
     homepage = "https://github.com/pop-os/cosmic-greeter";
     description = "Greeter for the COSMIC Desktop Environment";