{environment,readme}: default configuration path to /etc/nix-darwin

[emilazy]

Split off from The Plan stack, where it becomes more important, but a good idea anyway given our general direction and to align the default between flake‐ and channel‐based setups. This is not a breaking change thanks to the system.stateVersion conditional (unless anyone is looking at environment.darwinConfig on a flake‐based setup, which I guess is possible although seems inadvisable), so we can do it now.

[emilazy]

This is running into the thing where we test installation against the branch base. Not sure if we should fix that or what. (Testing upgrades seems like a good thing, but in this case the way it’s done is kind of annoying.)

[Samasaur1]

I definitely agree with the idea of the PR.

I don't feel completely confident in my understanding of the interaction of all these moving parts, though; what I read through seems good but I'm wondering if people will run into issues due to this change given the note on the environment.darwinConfig option:

NOTE: Changing this requires running darwin-rebuild switch -I darwin-config=/path/to/configuration.nix the first time to make darwin-rebuild aware of the custom location.

Given the stateVersion condition, I imagine nobody will run into issues unless they update their stateVersion, in which case perhaps they're just on their own?

[emilazy]

Yeah if you bump system.stateVersion without reading the changelog it's expected that stuff will explode.

[emilazy]

Realized that we were telling people to install 24.11 with the unstable darwin-rebuild. Whoops! Fixed.

[Enzime]

LGTM

Thanks for making this change, been on my list for so long 👍