rename `nixfmt-rfc-style`

[KiaraGrouwstra]

as per NixOS/nixpkgs#433100, the alias nixfmt-rfc-style was dropped, and the right name now as per NixOS/nixpkgs#486664 seems treefmt.
as such, we may want to adjust this over here as well.

[sandydoo]

We completed the switch when nixfmt was promoted to 1.0.

git-hooks.nix/nix/tools.nix

Lines 275 to 287 in 8baab58

	# nixfmt 1.0 is now the official Nix formatter as of 25.11.
	#
	# In 24.05, the `nixfmt` package was deprecated and replaced with two separate packages:
	# - nixfmt-classic
	# - nixfmt-rfc-style
	#
	# Remove this block in 26.05
	nixfmt =
	if lib.versionOlder nixfmt.version "1.0" && (nixfmt-classic.meta.isPlaceholder or false) then
	nixfmt-classic
	else
	nixfmt;
	inherit nixfmt-classic nixfmt-rfc-style;

Once 26.05 is out, we'll fully remove the hook and all the backwards-compat logic.