added firacode font family

pkgs/kitty/biscuit.nix

@@ -4,5 +4,6 @@
     confirm_os_window_close = 0;
     window_padding_width = 10;
     font_size = 12.0;
+    font_family = "firacode";
   };
 }

system/font.nix

@@ -1,5 +1,5 @@
 {pkgs, ...}: {
   fonts.packages = with pkgs; [
-    nerdfonts
+    nerd-fonts.fira-code
   ];
 }