Add official formatter to flake

Allows running `nix fmt`, which will format using the official style as
agreed upon in nixpkgs

flake.nix

@@ -87,6 +87,8 @@
 
           checks = builtins.mapAttrs (system: deployLib: deployLib.deployChecks self.deploy) inputs.deploy-rs.lib;
 
+          formatter.x86_64-linux = inputs.nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;
+
           deploy.nodes = self.lib.deploy.mkDeployNodes self.nixosConfigurations {
             nachtigall = {
               hostname = "10.7.6.1";