home: vim: null-ls: fix 'nixpkgs-fmt' condition

I haven't configured `nil` to format anything.

And I don't really care to, this is good enough for me.

modules/home/vim/plugin/settings/null-ls.lua

@@ -31,10 +31,8 @@ null_ls.register({
 -- Nix
 null_ls.register({
     null_ls.builtins.formatting.nixpkgs_fmt.with({
-        -- Only used if available, but prefer LSP if available
+        -- Only used if available
-        condition = function()
+        condition = utils.is_executable_condition("nixpkgs-fmt"),
-            return utils.is_executable("nixpkgs-fmt") and not utils.is_executable("nil")
-        end,
     }),
 })