nixos/installer: drop the extra nixUnstable in nixos-install

The only nix version available in the installer should be the version
configure in the module system. If someone needs `nixUnstable` in their
`nixos-install` they should probably set the module option and not just
add it to the closure.

nixos/modules/installer/tools/tools.nix

@@ -23,7 +23,6 @@ let
     inherit (pkgs) runtimeShell;
     nix = config.nix.package.out;
     path = makeBinPath [
-      pkgs.nixUnstable
       pkgs.jq
       nixos-enter
     ];