keybase: install the keybase package

(cherry picked from commit 9686d93ff6)

modules/services/keybase.nix

@@ -16,6 +16,8 @@ in
   };
 
   config = mkIf cfg.enable {
+    home.packages = [ pkgs.keybase ];
+
     systemd.user.services.keybase = {
       Unit = {
         Description = "Keybase service";