cgtcalc: auto-update from the default branch

Upstream doesn't have any useful tags.

Note that updating past the current rev will fail to compile until
nixpkgs gets swift 6 support:
https://github.com/NixOS/nixpkgs/issues/343210

But _not_ specifying this leads to confusing PRs:
https://github.com/NixOS/nixpkgs/pull/498904

pkgs/by-name/cg/cgtcalc/package.nix

@@ -45,7 +45,9 @@ stdenv.mkDerivation (finalAttrs: {
   # https://github.com/NixOS/nixpkgs/issues/379859
   doCheck = false;
 
-  passthru.updateScript = nix-update-script { };
+  passthru.updateScript = nix-update-script {
+    extraArgs = [ "--version=branch" ];
+  };
 
   meta = {
     description = "UK capital gains tax calculator written in Swift";