Currently, the `bin` directory of the configured system
is embedded in the `$PATH` of activation scripts, but not
other elements of the default `environment.systemPath` like
`/nix/var/nix/profiles/default/bin` or `/usr/local/bin`. This
means that when nix-darwin is not managing the Nix installation,
activation scripts like Home Manager’s that want to look up the
system‐managed Nix can’t find it. Search for it on the entire
`environment.systemPath` and add the appropriate directory if found.
We leave the launchd `activate-system` daemon alone, because it has
erroneously referred to `@out@/sw/bin` forever and therefore never got
a Nix on the path to begin with. That’s a problem for another time.
(The more ideal solution is probably for Home Manager activation to
be driven by launchd or something, but that’s a longer‐term goal.)
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.
