This is an equivalent of the `nix.enable` option from NixOS
and Home Manager. On NixOS, it mostly serves to allow building
fixed‐configuration systems without any Nix installation at
all. It should work for that purpose with nix-darwin too, and the
implementation is largely the same, but the main use case is more
similar to the Home Manager option: to allow the use of nix-darwin
with an unmanaged system installation of Nix, including when there
is another service expecting to manage it, as with Determinate.
By providing an escape hatch to opt out of Nix management entirely,
this will also allow us to consolidate and simplify our existing Nix
installation management, by being more opinionated about things like
taking ownership of the daemon and the build users. Porting one option
from NixOS lets us drop two that only ever existed in nix-darwin and
reduce overall complexity.
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