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>

fun/solves-this/default.nix

@@ -1,10 +1,12 @@
 { pkgs, ... }:
 let
-  mkWebroot = title: imgsrc: pkgs.runCommand "webroot" { } ''
-    mkdir -p $out
-    title="${title}" substituteAll ${./index.html} $out/index.html
-    cp ${imgsrc} $out/solves-this.png
-  '';
+  mkWebroot =
+    title: imgsrc:
+    pkgs.runCommand "webroot" { } ''
+      mkdir -p $out
+      title="${title}" substituteAll ${./index.html} $out/index.html
+      cp ${imgsrc} $out/solves-this.png
+    '';
 
 in
 {