darwin: support new system activation

[isabelroses]

nix-darwin is overhauling its system management to allow for multi user configuration. This will impact nh.

see: LnL7/nix-darwin#1341

[viperML]

We should ping the darwin maintainer, as I have no idea what nh darwin does @ToyVo

[khaneliman]

Current message output from existing implementation:

> Activating configuration for user
warning: `activate-user` is deprecated and will be removed in 25.11
This is usually due to the use of a non‐standard activation/deployment
tool. If you maintain one of these tools, our advice is:

  You can identify a post‐user‐activation configuration by the absence
  of `activate-user` or the second line of the script being
  `# nix-darwin: deprecated`.

  We recommend running `$systemConfig/sw/bin/darwin-rebuild activate`
  to activate built configurations; for a pre‐user‐activation
  configuration this should be run as a normal user, and for a
  post‐user‐activation configuration it should be run as `root`.

  If you can’t or don’t want to use `darwin-rebuild activate`, then you
  should skip running `activate-user` for post‐user‐activation
  configurations and continue running `activate` as `root`.

  In 25.11, `darwin-rebuild` will stop running `activate-user` and this
  transition script will be deleted; you should be able to safely
  remove all related logic by then.

Otherwise, you should report this to the deployment tool developers. If
you don’t use a third‐party deployment tool, please open a bug report
at <https://github.com/LnL7/nix-darwin/issues/new> and include as much
detail about your setup as possible.
> Activating configuration
setting up groups...