nix: error build user group has mismatching gid

[zimeg]

happening with changes of the ongoing update-

error: Build user group has mismatching GID, aborting activation
The default Nix build user group ID was changed from 30000 to 350.
You are currently managing Nix build users with nix-darwin, but your
nixbld group has GID 30000, whereas we expected 350.

Possible causes include setting up a new Nix installation with an
existing nix-darwin configuration, setting up a new nix-darwin
installation with an existing Nix installation, or manually increasing
your `system.stateVersion` setting.

You can set the configured group ID to match the actual value:

    ids.gids.nixbld = 30000;

We do not recommend trying to change the group ID with macOS user
management tools without a complete uninstallation and reinstallation

[zimeg]

updating configuration.nix lets builds happen but packages weren't updating so much

{
  ids = {
    gids = {
      nixbld = 30000;
    };
  };
}

[zimeg]

reinstalling nix has been so far a better solution 🙏

[zimeg]

unmounting /nix/store caused some problems: https://iboysoft.com/howto/fix-couldnt-unmount-disk-69888.html

the volume on disk0 couldn't be unmounted because it is in use by process 0.

fixing this uses these commands and a reboot!

$ diskutil list
$ sudo fuser -c /dev/identifier  # Replace "identifier" with the "Nix Store" disk
$ kill [PID]