nixos-rebuild-ng: only propagate nix's bin output

Prior to this the `dev` output was also propagated, when it's not
actually used

```console
$ nix-store --query --references /nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0
/nix/store/11ciq72n4fdv8rw6wgjgasfv4mjs1jrw-bash-5.2p37
/nix/store/26yi95240650jxp5dj78xzch70i1kzlz-python3-3.12.9
/nix/store/xxh7mivp64xmzyw5wir2c2xbhy6cjzjd-nix-2.24.12
/nix/store/8jai5cxdfzgj9nsz4i26fh9sx5zsgilz-nix-2.24.12-dev
/nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0
```

```console
$ nix why-depends --all --precise /nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0 /nix/store/8jai5cxdfzgj9nsz4i26fh9sx5zsgilz-nix-2.24.12-dev
/nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0
└───nix-support/propagated-build-inputs: …/nix/store/8jai5cxdfzgj9nsz4i26fh9sx5zsgilz-nix-2.24.12-dev /nix/store/26yi…
    → /nix/store/8jai5cxdfzgj9nsz4i26fh9sx5zsgilz-nix-2.24.12-dev
```

```console
$ nvd diff /nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0 /nix/store/fqm81bhggzkqh7033np2z0jr8c0qrpbw-nixos-rebuild-ng-0.0.0
<<< /nix/store/ppw0flx4dbksxsnr84hq1gz4k0s0hpcq-nixos-rebuild-ng-0.0.0
>>> /nix/store/fqm81bhggzkqh7033np2z0jr8c0qrpbw-nixos-rebuild-ng-0.0.0
Version changes:
[C.]  #1  boehm-gc  8.2.8, 8.2.8-dev -> 8.2.8
[C*]  #2  nix       2.24.12, 2.24.12-dev, 2.24.12-man -> 2.24.12, 2.24.12-man
Removed packages:
[R.]  #1  nlohmann_json  3.11.3
Closure size: 66 -> 63 (1 paths added, 4 paths removed, delta -3, disk usage -1.9MiB).
```

pkgs/by-name/ni/nixos-rebuild-ng/package.nix

@@ -50,7 +50,7 @@ python3Packages.buildPythonApplication rec {
     # would silently downgrade the whole system to be i686 NixOS on the
     # next reboot.
     # The binary will be included in the wrapper for Python.
-    nix
+    (lib.getBin nix)
   ];
 
   postPatch = ''