Tracking issue for deprecating installer

[Enzime]

• Replace copying the default configuration with instructions in README, see Tracking issue for updating README #1155
• Change services.nix-daemon.enable to default to true and add sufficient checks to tell users to set it to false if they are using single-user mode, see nix-daemon: enable by default #1164
• Replace setting up darwin channel with instructions in README, see Tracking issue for updating README #1155
• Test nix run nix-darwin -- switch -I darwin-config=~/.nixpkgs/darwin-configuration.nix
• Replace installer with darwin-rebuild switch command in README
• Ensure /etc/synthetic.conf always contains run if sw_vers > 10.15

[Enzime]

Not sure if we should change darwin-rebuild to default <darwin> to the nix-darwin it comes from

If we do that, then we could even automate adding the darwin channel as part of activation, rather than requiring users to manually do it

[emilazy]

Wouldn’t that pin an exact commit when used through nix run?

[Enzime]

Only if the user doesn't have <darwin> configured in their channels

We could also add an option like nix.configureDarwinChannel = true; which runs:

$ nix-channel --add https://github.com/LnL7/nix-darwin/archive/master.tar.gz darwin