nixos: set system.stateVersion from the nixpkgs release, not version

The nixpkgs/nixos version includes a suffix like "pre-git" or
"pre676716.6f16e67b4921", which does not match the conventional
"XX.YY" format of system.stateVersion.

Unifying the format to "XX.YY" allows for (stricter) validation (see #317858),
and the introduction in 3a5ff9a68c was
only concerned with silencing warnings, so the addition of the "pre.*"
suffix into stateVersion was probably unintentional.

flake.nix

@@ -64,7 +64,7 @@
               boot.loader.grub.enable = false;
               fileSystems."/".device = "nodev";
               # See https://search.nixos.org/options?show=system.stateVersion&query=stateversion
-              system.stateVersion = lib.versions.majorMinor lib.version; # DON'T do this in real configs!
+              system.stateVersion = lib.trivial.release; # DON'T do this in real configs!
             })
           ];
         }).config.system.build.toplevel;