style: Switch to nixfmt from nixpkgs-fmt

Most of the ecosystem has moved to this formatter,
and many people configured their editors to autoformat it with this formatter.

Closes: https://git.snix.dev/snix/snix/issues/62
Change-Id: Icf39e7836c91fc2ae49fbe22a40a639105bfb0bd
Reviewed-on: https://cl.snix.dev/c/snix/+/30671
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Autosubmit: Ilan Joselevich <personal@ilanjoselevich.com>

third_party/lix_forgejo/default.nix

@@ -1,6 +1,7 @@
 # Patches that the Lix core team developed for git.lix.systems
 # Re-applied for git.snix.dev
-{ ... }: {
+{ ... }:
+{
   patches = {
     # Show a link to upstream for a nixos/nix repository.
     upstream_link = ./upstream-link.patch;