skip using deprecated alias pkgs.nixUnstable

[yysushi]

pkgs.nixUnstable seems deprecated, so use pkgs.nixVersions.latest instead.

otherwise the following error occurs during bootstrap0. #40

     1574|         || pred here (elemAt values 1) (head values) then
     1575|           head values
         |           ^
     1576|         else

   … while evaluating the attribute 'value'

     at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/modules.nix:809:9:

      808|     in warnDeprecation opt //
      809|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
         |         ^
      810|         inherit (res.defsFinal') highestPrio;

   (stack trace truncated; use '--show-trace' to show the full trace)

   error: nixVersions.unstable has been removed. For bleeding edge (Nix master, roughly weekly updated) use nixVersions.git, otherwise use nixVersions.latest.