Preserve nix-darwin activation script PATH when activating.

[shlevy]

Description

If the system has a restrictive default PATH in sudoers, Nix might not be available within the activation script. Since our script sets its own PATH (except for inherting Nix), this should only impact looking up Nix itself.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[shlevy]

Note that this commit branched off from the common ancestor of master and 25.05 so that it can be easily backported if merged (instead of squashed/rebased)