Revert "Merge pull request #48122 from zimbatm/pkg-nixos-rebuild"

This reverts commit 10addad603, reversing
changes made to 7786575c6c.

NixOS scripts should be kept in the NixOS source tree, not in
pkgs. Moving them around is just confusing and creates unnecessary
code/history churn.

nixos/modules/misc/version.nix

@@ -43,7 +43,6 @@ in
     nixos.codeName = mkOption {
       readOnly = true;
       type = types.str;
-      default = lib.trivial.codeName;
       description = "The NixOS release code name (e.g. <literal>Emu</literal>).";
     };
 
@@ -80,6 +79,9 @@ in
       version = mkDefault (cfg.release + cfg.versionSuffix);
       revision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
       versionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));
+
+      # Note: the first letter is bumped on every release.  It's an animal.
+      codeName = "Koi";
     };
 
     # Generate /etc/os-release.  See