nixos/kanata: check the config file at build time

Before this patch, checking the config file is done at runtime.
Doing so at build time shortens the feedback loop[1][2].

[1]: https://github.com/NixOS/nixpkgs/issues/278135
[2]: https://github.com/jtroo/kanata/issues/689

nixos/modules/services/hardware/kanata.nix

@@ -86,7 +86,9 @@ let
     in
     optionalString ((length devices) > 0) "linux-dev (${devicesString})";
 
-  mkConfig = name: keyboard: pkgs.writeText "${mkName name}-config.kdb" ''
+  mkConfig = name: keyboard: pkgs.writeTextFile {
+    name = "${mkName name}-config.kdb";
+    text = ''
       (defcfg
         ${keyboard.extraDefCfg}
         ${mkDevices keyboard.devices}
@@ -94,6 +96,10 @@ let
 
       ${keyboard.config}
     '';
+    checkPhase = ''
+      ${getExe cfg.package} --cfg "$target" --check --debug
+    '';
+  };
 
   mkService = name: keyboard: nameValuePair (mkName name) {
     wantedBy = [ "multi-user.target" ];