fix: use nixfmt directly in nixvim to avoid format timeout

modules/applications/vim/home/default.nix

@@ -52,6 +52,7 @@ in
             stylua
             shfmt
             alejandra
+            nixfmt-rfc-style
             lua-language-server
             nixd
           ];

modules/applications/vim/home/plugins/editor/conform.nix

@@ -15,17 +15,7 @@ _: {
         sh = [ "shfmt" ];
       };
       formatters.nix_fmt = {
-        command = "nix";
+        command = "nixfmt";
-        args = [
-          "fmt"
-          "$FILENAME"
-        ];
-        stdin = false;
-        condition.__raw = ''
-          function()
-            return vim.fn.executable("nix") == 1 and vim.fn.filereadable(vim.fn.findfile("flake.nix", ".;")) == 1
-          end
-        '';
       };
       format_on_save.__raw = ''
         function(_)