nix: Update Nix formatter

Signed-off-by: Steffen Vogel <steffen.vogel@opal-rt.com>

flake.nix

@@ -25,7 +25,7 @@
           ];
         };
 
-        formatter = nixpkgs.alejandra;
+        formatter = nixpkgs.nixfmt-rfc-style;
       }
     );
 }