shell: avoid configuring bash if fish is used

this saves 1.34 MiB according to dix; why not!

defaults/shell.nix

@@ -47,7 +47,9 @@ in
   config = lib.mkIf cfg.enable {
     # use Fish, the nicest shell :)
     programs.fish.enable = lib.mkIf config.nixowos.allowAddingPackages true;
+
     users.defaultUserShell = lib.mkIf config.programs.fish.enable config.programs.fish.package;
+    programs.bash.enable = lib.mkIf config.programs.fish.enable false;
 
     environment.shellAliases = {
       # make nix-shell preserve the user's $SHELL