home: comma: configure custom 'nixpkgs' flake

This makes use of my pinned `pkgs` flake from the registry by default.

home/comma.nix

@@ -5,11 +5,25 @@ in
 {
   options.my.home.comma = with lib; {
     enable = my.mkDisableOption "comma configuration";
+
+    pkgsFlake = mkOption {
+      type = types.str;
+      default = "pkgs";
+      example = "nixpkgs";
+      description = ''
+        Which flake from the registry should be used with
+        <command>nix shell</command>.
+      '';
+    };
   };
 
   config = lib.mkIf cfg.enable {
     home.packages = with pkgs; [
       ambroisie.comma
     ];
+
+    home.sessionVariables = {
+      COMMA_PKGS_FLAKE = cfg.pkgsFlake;
+    };
   };
 }