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>

nix/readTree/tests/test-tree-traversal/default-nix/can-be-drv/default.nix

@@ -3,5 +3,8 @@ derivation {
   name = "im-a-drv";
   system = builtins.currentSystem;
   builder = "/bin/sh";
-  args = [ "-c" ''echo "" > $out'' ];
+  args = [
+    "-c"
+    ''echo "" > $out''
+  ];
 }

nix/readTree/tests/test-tree-traversal/here-arg/default.nix

@@ -1,4 +1,5 @@
-{ here, ... }: {
+{ here, ... }:
+{
   attr1 = "foo";
   attr2 = here.attr1;
 

nix/readTree/tests/test-tree-traversal/here-arg/subdir/sibl2.nix

@@ -1,2 +1 @@
-{ here, ... }:
-here.sibl1
+{ here, ... }: here.sibl1