pkgs: comma: rename 'COMMA_{,NIX}PKGS_FLAKE'

This aligns with the rust implementation of this tool [1].

[1]: 17a4f33849

home/comma/default.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_NIXPKGS_FLAKE = cfg.pkgsFlake;
+    };
   };
 }