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>

ops/secrets/mkSecrets.nix

@@ -12,10 +12,12 @@ let
   agePubkey = types.typedef "age pubkey" (s: isString s && hasPrefix "age" s);
 
   agenixSecret = types.struct "agenixSecret" {
-    publicKeys = types.listOf (types.union [
-      sshPubkey
-      agePubkey
-    ]);
+    publicKeys = types.listOf (
+      types.union [
+        sshPubkey
+        agePubkey
+      ]
+    );
   };
 
 in