Restore "nixUnstable: rename to nix-unstable"

as discussed in bed3695848

Different names make it easier for the users to notice updates to Nix stable,
and to have Nix stable and unstable simultaneously.

This reverts commit bed3695848.

pkgs/tools/package-management/nix/default.nix

@@ -160,7 +160,7 @@ in rec {
   }) // { perl-bindings = nixStable; };
 
   nixUnstable = (lib.lowPrio (common rec {
-    name = "nix-1.12${suffix}";
+    name = "nix-unstable-1.12${suffix}";
     suffix = "pre5788_e3013543";
     src = fetchFromGitHub {
       owner = "NixOS";