Reword BUILDING.md wrt nix support

Signed-off-by: Hanno Becker <[email protected]>

Author: hanno-becker

BUILDING.md

@@ -71,7 +71,9 @@ There are further scripts used for development of mlkem-native, such as `format`
 
 ### nix setup
 
-We specify the development environment for mlkem-native using nix. If you want to help develop mlkem-native, please setup nix using the [nix installer script](https://nixos.org/download/), not your package manager.
+We specify the development environment for mlkem-native using `nix`. If you want to help develop mlkem-native, please
+use `nix`. We recommend using the latest Nix version provided by the [nix installer
+script](https://nixos.org/download/), but we currently support all Nix versions >= 2.6.
 
 All the development and build dependencies are specified in [flake.nix](flake.nix). To execute a bash shell, run
 ```bash